Import functionality in fixture editor

For some of our plugins, we've already implemented an import method to create OFL fixtures out of a given foreign fixture file. This functionality is still only accessible via a CLI tool; there should be an easy tool in the public UI for this!

Possible workflow for an end-user:

1. Go to the import page.
2. Select a plugin.
3. Upload a fixture file (maybe also more than one; some plugins need several files to represent a single fixture)
4. Show which fixtures could be imported (maybe allow merging fixtures to handle different modes being represented by different fixtures?)
5. Finish definitions fixture by fixture: Show the fixture editor, prefilled with the imported data (similar to editing existing fixtures #564) and the fixture-valid result in the beginning.
6. Submit all fixtures as pull request (like adding a new fixture, but now just several ones).

Like editing existing fixtures, we have to deal with fixture features that aren't supported by the editor yet (see #311). Maybe we simply wait until the editor supports all features, or the fixtures with unsupported features are grayed out in 4. and need manual editing.

The issue names no files or tests. Start by tracing the existing CLI import method and the public fixture editor, then compare them with the workflows described here; done means users can select a plugin, upload one or more files, review and edit imported fixtures, and submit the resulting definitions as a pull request.