Add git integration to server

Adds git integration as in #71.

Let me know if I should squash all relevant commits into a single one.

The branch also contains some early commits of minor fixes I picked up along the way in my first few days exploring the codebase. Let me know if I should move them to a separate MR or remove them altogether.

scipost_django/SciPost_v1/settings/base.py

@@ -570,3 +570,8 @@ DISCOURSE_SSO_SECRET = get_secret("DISCOURSE_SSO_SECRET")
 CORS_ALLOWED_ORIGINS = [
     "https://git.scipost.org",
 ]
+
+# GitLab API
+GITLAB_ROOT = "SciPost"
+GITLAB_URL = "https://git.scipost.org/"
+GITLAB_KEY = get_secret("GITLAB_KEY")