the initial reason for this change was that graphicsmagick does not
support writing to ico files. this fact lead to a chain of changes:
1. use png instead of ico (browser support is good enough)
2. render the overlays on the client using the canvas API. this way we
only need to store the original favion and generate the overlay versions
dynamically.
this change also enables (next step) to simplify the handling of the
stock favicons as well, as we don't need to generate all the versions
upfront.
this ways we can keep the `lib/gitlab/ci/status/*` classes to return the
bare favicon name as it was before.
also the favicon uploader versions are now have the same names as the
stock favicons (+ `favicon_` prefix), which makes working with the
status names easier.
