* Rework 'make generate-images'
- Remove external dependencies and replace it with a node script that
does does the same.
- Move detail removal from gitea-sm.png to favicon.png
- Remove favicon.ico and its generation, it is unused and we already serve
favicon.png in its place.
Fixes: https://github.com/go-gitea/gitea/issues/12314
* use proper centering value for preserveAspectRatio
* fix lint
* use fabric
* better linting fix
* fix typo
* mention detail-remove class in docs
Also changes the avatar from a jpeg to a png, to allow for
transparent background. The indexed png is also smaller in size.
Note that at the moment the default avatar is only used when
the user requested a custom avatar and the custom avatar file
is not found (should never happen).
In the future the default avatar could be used as a default
return when by-mail avatar lookups fail too (both gravatar
and libravatar support passing a default)
