Improve discoverability of rich text editor

Problem to solve

After a successful "beta" period, the Content Editor usage has dropped off significantly starting in December of 2021. (See internal Sisense dashboard)

Several root causes may be contributing to this drop, but one theory is that when we moved away from the prominent "banner" treatment, highlighting the new editor, we made the CTA to toggle between editors too subtle.

Proposal

In order to improve discoverability, re-visit the visual treatment of the toggle between Source / Content editing on the Wiki.

Current UI	Proposal

Implementation guide

NOTICE: Limit the changes in the MR that resolves this issue to replacing the current switch control with the segmented control proposed above.

• Use the segmented control component to implement this change.
• Keep the structure the same. We should still have the toggle live outside the editing area itself, as the toggle replaces everything below it.

Further steps after delivering this change

1. We may need to re-open the discussion about what to call the WYSIWYG mode. Edit rich text might not be as intuitive as we thought.
2. Lightweight user testing will likely be necessary to validate whatever changes we make.

Availability and Testing

Changes to tests required. Please run package-and-qa and assign a SET to review any MRs.