Add TextFormatter support (attempt 2)

[dsevillamartin]

Fixes #37
Fixes #38
Supersedes #42

Changes proposed in this pull request:

• Trim unparsed bio before saving to DB (content provided by user)
• Use toned-down TextFormatter instance (with markdown and/or bbcode if those extensions are enabled)
  • No images, iframes, or embeds. This formatter isn't extended through Flarum's Formatter extender, so shouldn't be issues with other exts accidentally adding stuff
• Do not raise BioChanged event if bio did not change\

Reviewers should focus on:

• I think its fine to reuse the storage/formatter folder as the class files are different.
• The highlight.js code doesn't run and I don't know why nor really care 🤷

Screenshot

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).

[dsevillamartin]