Finish removing forced user info update.
Fix theme issues.

* Light theme keyboard is now light.
* PM compose view's to and subject text fields have visible text in dark theme.
* Ditto thread compose view's subject field.
Don't crash on missing images for thread tags.
Remove hpple license and acknowledgement.

No longer using the library.
Use PSMenuItem pod instead of manual inclusion.

Didn't notice there was an unofficial podspec!
Crash harder on text view content offset weirdness.
Ask to save or delete ongoing drafts of edited posts.

Previously if you started an edit then pressed "Cancel", you'd still be editing
if you pressed "Compose" without leaving the thread. While the submit button's
text would be "Save", not "Post", it wasn't clear that you were picking up the
edit from where you left off, and it was easy to accidentally make your new
post an edit of some other one.

By showing a confirmation sheet we hopefully accomplish two things:

1. You can stop editing the post if you want. (Previously you'd have to leave
   the thread to do this.)
2. If you choose to keep the edit around, you're warned that that's what you're
   doing.