Unify button styles (Part 1)

[mmmavis]

Related to #2905 (To-be-filed Part 2 PR will close it)

Part 1 Work (this PR)

Changes

• addressed everything mentioned in Unify button styles #2905 (expect the Button Arrangement part as that part is best practice for designers and CMS editors)
• class LinkButtonBlock: added new button options & updated all instances of that block use via context
• we have decided to have .btn- as the classname prefix (instead of .button- as documented in the original ticket)

To code reviewer

Please focus on changes in the following files. Changes in other files are just find-and-replace class name updates.

• .scss files,
• network-api/networkapi/wagtailpages/customblocks.py,
• and network-api/networkapi/wagtailpages/templates/wagtailpages/blocks

Part 2 Work (as follow-up #2936)

• remove old options btn-normal and btn-ghost from class LinkButtonBlock

[mmmavis]

@sabrinang this is ready for final design review. Please verify changes on review app as well as confirm the UI differences Percy flagged are all intended.

[mmmavis]

I will assign people for code review once this has passed design review.

[sabrinang]

class names, hover states, icon transition and colours look good 👍
(using sentence case for button labels is also a best practice for CMS editors)

[Pomax]

Just a few requests

PR updated

[Pomax]

💥 🚀 👾 💯