Support default timezone, specified globally in config and remembered in account tags for account

[elenaustyugova]

Acceptance criteria:

1. It should be possible to specify a default timezone in config.json. This timezone should be preselected when a user opens a list of zones (e.g. when creating a user)
2. If a user chooses another timezone (not the default one), store this zone in account tags (consult Store "SSH description" in account tags if supported #824 as an example).
3. If the zone was saved to account tags, it should be preselected in a list when user opens this dialog next time instead of a default one.
4. Document this feature in Config Guide and a local "Custom Features" document
5. Add a tag to wiki tags page

Connected feature ID:

• timezone_default_config
• account_user_create