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


Regarding the expire time I would choose the default setting from the remote site. We might get this by not specifying the expiry?

In regards of FormData vs. MultipartFormData we should extend the latter module/class so it checks if FormData  is available and uses it. Otherwise it can handle the request with our custom code. I would not introduce another module for that, given that we want to get rid of it soon.