It adds a brand new importer for Gitea!

This is a continuation of !6945 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.

## Are there points in the code the reviewer needs to double check?

1. 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.
1. I've added a base `IssuableFormatter` class from which `IssueFormatter` & `PullRequestFormatter` inherit
1. I've added shared examples for GitHub/Gitea importer classes
1. I've made `Gitlab::ImportSources` more robust and tested! 🎄 
1. I've added routing specs for import routes! 🎄 

Closes #22348

See merge request !8116