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


> Whatever you feel. You can do it also as a separate PR before we finish off this PR. Smaller code changes are always better as huge PRs for me atm.

Ok, I will open a new PR about extending `MultipartFormData`.
I'd like to add a reference to it (the new PR) pointing to #176.