Edit existing fixtures in fixture editor

[fxedel]

Basically, the editor should be prefilled with the existing fixture data. Submitting the fixture will

• create a pull request to edit the fixture (when submitted from the main open-fixture-library.org deployment)
• add a commit to the current branch (when submitted from a preview deployment) → this allows fixture submitters to edit their fixture again (e.g. if they forgot something)

Maybe we can also allow JSON editing in the online editor (with validation) for more experienced / developer-minded people.

As long as the editor doesn't support all features from our schema yet, editing should only be possible for fixtures that don't use unsupported features. This can be ensured with #311.

It should also be possible to submit the data as new fixture (i.e. to use a similar fixture as starting point when adding a new one). Thanks to @dimitre for the idea (#224)!