Add SALSA_AVATAR_PATH

REF:

• GitLab API: https://docs.gitlab.com/ee/api/projects.html#upload-a-project-avatar

This can only be merged when bluefeet/GitLab-API-v4 #54 has been merged upstream and the package updated (libgitlab-api-v4-perl). GitLab CI will fail until then too!

This is because of GitLab requiring a PUT request, using multipart/form-data is missing from Perl's Tiny HTTP missing this functionality.

$ grep SALSA_AVATAR_PATH config.conf
SALSA_AVATAR_PATH=icon.png
$ perl scripts/salsa.pl --conf-file config.conf update_safe example --verbose --debug
$
$ perl scripts/salsa.pl --conf-file config.conf update_safe example --verbose --debug --avatar-path=icon.png
$ perl scripts/salsa.pl --conf-file config.conf update_safe example --verbose --debug --avatar-path=%p.png
$ perl scripts/salsa.pl --conf-file config.conf update_safe example --verbose --debug --avatar-path=no-file-here