feat: add support for text file preview embeds

[davwheat]

Changes proposed in this pull request:

• Adds a new text file preview upload template.

Reviewers should focus on:

Does this scale for large communities, since the file is loaded in JS? Somehow injecting the initial snippet in the template seems like a better idea, but I've no idea how that can be done.

Screenshot

Confirmed

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

[davwheat]

@clarkwinkelmann Still got the other things to work on but after hours of pain, I've got the snippet being injected with a callback.

[davwheat]

I'm going to need to somehow include the post ID in the formatted view, because otherwise sometimes the URL it attempts to fetch contains undefined.