Add interface button to clear image list cache

[dersmon]

The application caches an index of files and their size for each project to avoid unnecessary file system lookups. This cache by default is retained for 24h or until a new file gets added or an existing one deleted.

The addition/deletion deletes the cached file index, which will be recreated the next time someone asks for the index.

Currently there is no way for administrators to trigger the deletion through the web interface directly, which should be a possibility for certain edge cases: If files were added directly server side on file system level instead of through the application.

There already exists a helper function to clear the cache (currently declared as private), see https://github.com/dainst/idai-field/blob/master/server/lib/field_hub/file_store.ex#L323-L325

[dersmon]

#189

[the-last-pastafarian]

#189