[API] Add already fetched videos to localStorage after every request

[NikkelM]

This prevents users that click the random button multiple times from eating up API queries, by re-using what was already fetched

[NikkelM]

If this becomes necessary we can think about it, otherwise the amount of complexity this would add to the fetching logic outweighs the benefits

[NikkelM]

We could use an entry in the localStorage for "inProgress" playlists, which would also store items such as the most recent pageToken etc. to continue where fetching was last stopped