Add setting to only opt-in selected media bundles

I feel like the table approach is a bit too much, as it takes a lot of space for a single setting. I feel like the better approach would be, to use a multiselect here, but @Anybody should decide.

Postponed on #3271249: Only handle known providers (whitelist) and exclude others → which should definitely be solved first. In general I think the settings and rebuild concept should be overthought before.

Note, that the rebuild checksum is only needed, if entities exist BEFORE the installation of this module. If the module is already installed, newly added duplicates will get automatically tracked on the reports page.

To properly handle this, my suggestion would be to exclude specified media types on the lowest level of the module inside "ChecksumStatistics". This is the main class used for checksum related database queries, where duplicate checksums, checksum existence, number of missing checksums, etc. is being checked. I'd say, we exclude the media types inside the queries there.

Just turned around the title.

We should add a setting to only include the selected media types. If none is selected (default) all media types are checked!

@grevil: We have this case now. Could you finish this, like described in #13 please? Feel free to delegate to LRwebks, if it makes sense. Basic work has already been done, but needs turn-around and finishing.