Pr 3.2.18 restore sending as uploads, and a few other things

Sending stickers as uploads now work the same way they used to.
Improve logic of determining selected textarea, channel, and all that jazz.
Magane button no longer appearing in textareas of Create Thread and Create Forum Post forms.
They'd never work there to begin with, since such textareas would've had no active "channel" until they had been created.
Also add metadata comments for Vencord plugin file. Wanted to expand the update checker for Vencord as well, but it seems like it'll require a bit more experimenting but I'm out of time. I'll probably get back to it at another day so I'm finalizing this PR already.

Pitu / Magane