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


> We might get this by not specifying the expiry?

The default expiry what p.m.o offers is `d` as day, which is "Good for IRC or IM conversations".
I vote for `m` as month: "Good for email conversations / temporary data".

> ... we should extend the latter module/class so it checks if FormData is available and uses it.

Should this PR cover that extension of `MultipartFormData`?