Feat/text formatter

[n-peugnet]

Closes #4

Still missing:

• [x] back reference with #<id> links
• [x] invalidate tag in front-end if discussion does not exist
• [x] dynamically load discussion titles in front-end. Blocked by https://github.com/flarum/framework/issues/3720 (workaround in 75e4a653d487a1f5e3aa6273dd5f0f96f04d25c4)
• [x] cache store error responses to avoid querying multiple times the same unavailable discussion

I really don't know how to do the last two ones as TextFormatter js filters are run synchronously while the store interface is asynchronous.