SPIKE: Investigate alternatives to TinyMCE

[GuySartorelli]

TinyMCE 7 changed its license to GPLv2+, which isn't compatible with our licenses, so we need to swap to a new editor.

A very quick look was given to some popular editors in silverstripe/.github#334 (comment) but it was far from extensive, and doesn't give us enough information to choose one to go forward with.

We need to investigate various editors so that we have enough information to make a decision about what editor to use going forward.

Specific things to look for

This list is not exhaustive, but includes some things that we should specifically investigate

• extensibility - can we easily add plugins to add new functionality? e.g. specific types of links, insert images with our own options, shortcode support, etc
• user experience - will the user experience feel familiar? Or will content authors need to re-learn how to use the WYSIWYG?
• feature set - are most of the features content editors want already implemented? lists, tables, general rich text editing, ability to see and edit RAW HTML
• Are we able to provide developers a flexible way to allow-list specific HTML elements and attributes in a way that is comparable to the TinyMCE config?
• UI - Is there a UI available (either officially or third-party)? Is the UI easily extensible (can we add our own buttons/menu items/etc)?