Change links from 5-3 branch to point to master instead

Fix for #4535 ('Test settings' button for services gives 500 on empty repo)

new animated-screenshots

Clarify documentation on how to add custom Omniauth provider to GitLab

- half size: 1465 KB -> 641KB
- mutch better quality
- no MakeAGIF.com branding
- HTTPS link (via github gist) so there is no github-camo proxy needed (camo makes this image loading 0.55s slower in my short test)
- all source-files are available here: https://gist.github.com/fnkr/2f9badd56bfe0ed04ee7