Flexible import_projects

We have a project that looks something like:

```
po/*/messages.po
plugin/foo/*/messages.po
plugin/foo/*/javascript.po
plugin/bar/*/messages.po
plugin/bar/*/javascript.po
```

So, I wanted to mass import all the plugins, using the main app's shared repository.
I also needed to be able to many of the plugins twice: messages and javascript.

So, I did a few things:
* Allow mass importing with a `weblate://` URL
* Skip existing sub-projects when mass importing.
* Allow customising the project name when mass importing (so I could append `JS` for `javascript.po` imports)

While I was working on it, I noticed that:
* Nothing enforced uniqueness of project/sub-project name/slugs.
* `RepoTestCase` would break as it never deleted the repos from `GIT_ROOT`.
* Mass imported projects' names weren't slugified.