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


> Can we make use of FormData here?

Should I modify `MultipartFormData` to make it a wrapper around `FormData` if it exists?
Or simply make a new `nttFormData.jsm` which uses `FormData` if available, and `MultipartFormData` otherwise?
I vote for the `nttFormData.jsm` way.