Make src.fedoraproject.org repos searchable on sourcegraph.com

Created by: tsenart

Context

As part of the search core team's Q4 OKR regarding adding a wider range of code hosts to the global search index, we're going to sync src.fedoraproject.org repos on sourcegraph.com.

Work

• Implement PagureSource in repo-updater + UI + docs (3 days)
• Set user agent of repo-updater sources to Sourcegraph-Bot (0.5 days)
• Extend ListIndexable SQL query to return any repositories from src.fedoraproject.org (0.5 days)
• Set git user agent in gitserver to Sourcegraph-Bot (0.5 days)
• Figure out why syntect_server fails highlighting for some .spec files (???)