Fix projects mode bugs (#29593)

Fix for regressions introduced by #28805 

Enabled projects on repos created before the PR weren't detected. Also,
the way projects mode was detected in settings didn't match the way it
was detected on permission check, which leads to confusion.

Co-authored-by: Giteabot <teabot@gitea.io>

models/fixtures/repo_unit.yml

@@ -520,7 +520,6 @@
   id: 75
   repo_id: 1
   type: 8
-  config: "{\"ProjectsMode\":\"all\"}"
   created_unix: 946684810
 
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