Fix UniqueUsernameValidator comparison

Comparison was downcasing only one side, therefore if previously
existing account had a non-lowercase spelling, it would be ignored
when checking for duplicates.

New rake task `mastodon:maintenance:find_duplicate_usernames` will
help find constraint violations that might have occured from the
presence of this bug.

Bump version to 2.3.3

Bump version to 2.3.2

Bump version to 2.3.2rc5

Bump version to 2.3.2rc4

Bump version to 2.3.2rc3

Bump version to 2.3.2rc2

Revert "Upgrade Paperclip to version 6.0.0" (mastodon#6807)

* Revert "Bump version to 2.3.2rc1"

This reverts commit cdf8b92.

* Revert "Downgrade Dockerfile to Ruby 2.4.3 on Alpine 3.6 (mastodon#6806)"

This reverts commit 0074cad.

* Revert "Handle Mastodon::HostValidationError when pulling remoteable assets (mastodon#6782)"

This reverts commit 4a0a19f.

* Revert "Correct the reference to user's password in mastodon:add_user task (mastodon#6800)"

This reverts commit 338bff8.

* Revert "Upgrade Paperclip to version 6.0.0 (mastodon#6754)"

This reverts commit b88fcd5.

Weblate translations (2018-03-13) (mastodon#6777)

* Translated using Weblate (Portuguese (Brazil))

Currently translated at 100.0% (280 of 280 strings)

Translation: Mastodon/React
Translate-URL: https://weblate.joinmastodon.org/projects/mastodon/frontend/pt_BR/

* Translated using Weblate (Spanish)

Currently translated at 99.6% (579 of 581 strings)

Translation: Mastodon/Backend
Translate-URL: https://weblate.joinmastodon.org/projects/mastodon/backend/es/

* Translated using Weblate (Spanish)

Currently translated at 100.0% (58 of 58 strings)

Translation: Mastodon/Preferences
Translate-URL: https://weblate.joinmastodon.org/projects/mastodon/simple_form/es/

* Translated using Weblate (Japanese)

Currently translated at 100.0% (280 of 280 strings)

Translation: Mastodon/React
Translate-URL: https://weblate.joinmastodon.org/projects/mastodon/frontend/ja/

* Translated using Weblate (French)

Currently translated at 100.0% (280 of 280 strings)

Translation: Mastodon/React
Translate-URL: https://weblate.joinmastodon.org/projects/mastodon/frontend/fr/

* Translated using Weblate (Spanish)

Currently translated at 100.0% (58 of 58 strings)

Translation: Mastodon/Preferences
Translate-URL: https://weblate.joinmastodon.org/projects/mastodon/simple_form/es/

* Translated using Weblate (Indonesian)

Currently translated at 100.0% (280 of 280 strings)

Translation: Mastodon/React
Translate-URL: https://weblate.joinmastodon.org/projects/mastodon/frontend/id/

* Translated using Weblate (Spanish)

Currently translated at 100.0% (280 of 280 strings)

Translation: Mastodon/React
Translate-URL: https://weblate.joinmastodon.org/projects/mastodon/frontend/es/

* Translated using Weblate (Indonesian)

Currently translated at 94.6% (71 of 75 strings)

Translation: Mastodon/Doorkeeper
Translate-URL: https://weblate.joinmastodon.org/projects/mastodon/doorkeeper/id/

* Translated using Weblate (Indonesian)

Currently translated at 100.0% (280 of 280 strings)

Translation: Mastodon/React
Translate-URL: https://weblate.joinmastodon.org/projects/mastodon/frontend/id/

* Translated using Weblate (Arabic)

Currently translated at 100.0% (62 of 62 strings)

Translation: Mastodon/Devise
Translate-URL: https://weblate.joinmastodon.org/projects/mastodon/devise/ar/

* Translated using Weblate (Indonesian)

Currently translated at 100.0% (280 of 280 strings)

Translation: Mastodon/React
Translate-URL: https://weblate.joinmastodon.org/projects/mastodon/frontend/id/

* Translated using Weblate (Dutch)

Currently translated at 100.0% (280 of 280 strings)

Translation: Mastodon/React
Translate-URL: https://weblate.joinmastodon.org/projects/mastodon/frontend/nl/

* Translated using Weblate (Arabic)

Currently translated at 75.5% (439 of 581 strings)

Translation: Mastodon/Backend
Translate-URL: https://weblate.joinmastodon.org/projects/mastodon/backend/ar/

* Translated using Weblate (Arabic)

Currently translated at 76.2% (443 of 581 strings)

Translation: Mastodon/Backend
Translate-URL: https://weblate.joinmastodon.org/projects/mastodon/backend/ar/

* Translated using Weblate (Russian)

Currently translated at 95.8% (557 of 581 strings)

Translation: Mastodon/Backend
Translate-URL: https://weblate.joinmastodon.org/projects/mastodon/backend/ru/

* Normalize translations
Ran i18n-tasks normalize && yarn manage:translations

Bump version to 2.3.1rc3

Bump version to 2.3.1rc2