Replace Trix with Lexxy

[monorkin]

Lexxy is an ActionText frontend replacement for Trix that has a few advantages:

1. It supports WYSIWYG markdown editing (closes Markdown messages support #36)
2. It supports inline and block code blocks with syntax highlighting

While the Ruby side of replacing Trix with Lexxy is straight forward, a lot of the Stimulus controllers will have to be updated to work with Lexxy - like the composer, the unfurler, ...

[sethwbarton]

Hi! I'd like to take a shot at this if no one else has it in progress.

It looks like the result of #36 is that we want the full Lexxy experience instead of just putting in markdown support alone. So I'll give it a shot. I'm new to this project so hopefully I get all the controllers that need to get updated.

[milos-dukic]

One question regarding the UI/UX, should new Lexxy input look & feel, be the same as current Trix?