DText header IDs may hijack reserved IDs

[evazion]

Header IDs can be abused to hijack IDs already in use by the site:

h1#artist-commentary. Look at me! - style a comment obnoxiously.

h1#add-to-favorites. nope - break the 'F to favorite' hotkey (may need to put this in a flag message to ensure it's high enough in the HTML to hijack the $("#add-to-favorites") javascript).

I suggest that header IDs must follow a safe format that won't interfere with site IDs. Maybe something like h1#dtext-blah-blah. or h1#header-blah-blah.

[r888888888]

Alternatively (and this may not be any more obvious) when the id is converted to HTML, always prepend some prefix so there's no chance of clashing.

[Type-kun]

https://danbooru.donmai.us/forum_posts/119633 suggest changing dtext- prefix to something more descriptive. Either way, old dtext.rb and corresponding test for it should be fixed to reflect this, too.