TLDR: GitHub gist feature for Gitea

Long Version:

Add a new repo type named "Gist" and show them in the webui like a GitHub Gist:

• show all files as previews on one page
• have a comment section below
• only one branch exists (pre-push hook to block everything else)
• show a tab with commits/revisions not with as list of commits but with a list of diff-views
• allow forks but not PRs

Proposals for implementation:

• URL: <base-url>/<user_or_organization>/gists/<uuid> (gists must be a blacklisted repo name)
• save gist as repo (add a RepoType field to database)
• adjust routing to first check for gists and go to gist handling; else go to user repo handling
• add new comment type for gist comments