Resolve "Test GitHub importer with Gogs" (!8116) · Merge requests · GitLab.org / GitLab FOSS

What does this MR do?

It adds a brand new importer for Gitea!

This is a continuation of !6945 (closed) started by @bkc.

Gitea aims to be 100% GitHub-compatible but there's a few differences:

Gitea is not an OAuth provider (yet): https://github.com/go-gitea/gitea/issues/27
- This means we cannot map Gitea users given an assignee ID => assignees are not set on imported issues and merge requests
No releases API for now: https://github.com/go-gitea/gitea/issues/330
API version is v1 (GitHub is v3)
The IID field for milestones is id compared to number in GitHub.
Issues, PRs, milestones, labels don't have a url field (the importer now fallback to '' in that case)

Known issues:

Comments are not imported because comments JSON always have a blank html_url/issue_url/pull_request_url, so the IID cannot be extracted and the issuable cannot be found... :( This is tracked in https://github.com/go-gitea/gitea/issues/401, and solved by https://github.com/gogits/gogs/pull/3624 but this needs to be submitted / merged in Gitea.

This is noted in the documentation:

I've made Import::GiteaController inherit from Import::GithubController since both controllers should be identical in the long-term and their current differences are small.
I've added a base IssuableFormatter class from which IssueFormatter & PullRequestFormatter inherit
I've added shared examples for GitHub/Gitea importer classes
I've made Gitlab::ImportSources more robust and tested! 🎄
I've added routing specs for import routes! 🎄