Bug 977355 - broken link to superheroes wanted on mozilla.org/contribute

Update a Thunderbird KB link along with
https://github.com/mozilla/bedrock/pull/1738.
Merge pull request #273 from kyoshino/bug-977355-tb-support-links

Bug 977355 - broken link to superheroes wanted on mozilla.org/contribute
Updating donate button links
Merge pull request #274 from retornam/fixdonate

Updating donate button links
Moved paragraph on installing Python libraries

So that Ubuntu/Debian users know what they should do first upfront.
Merge pull request #276 from pablofiu/patch-1

Moved paragraph on installing Python libraries
updating contribute tests
Merge pull request #278 from retornam/contribs

updating contribute tests
fixing notes header locator
Merge pull request #279 from retornam/notesfix

fixing notes header locator