Use pastebin.m.o with FormData where applicable. Fixes #86.

This is a pull request to deal with the broken feature - send `about:support` to Pastebin. (#86)


I made other improvements too: 
 * disable menuitem while sending to avoid multiple pastes
 * hide menuitem if there isn't any `about:support` page
 * Thunderbird support - a little ugly modification was enough


Hmmm. What happened with my answer?

> Not sure what to answer right now.

Q: "How long should your post be retained?"
A:  one of the following:
* a day
* a month
* forever 

> Can we make use of FormData here?

This PR uses `FormData` if available, and falls back to NTT's builtin `MultipartFormData` if its missing.