Make calculating most popular items more efficient

[tjenkinson]

With the changes in #583 it should now be possible to make calculating the most popular items more efficient.

The current way is inefficient because previously there was no nice way due to how the view counts were stored.

It should now be possible to make a single nice query on the playback_history table to get the items ordered by the amount of view counts they have.

Before this can happen though the current values in initial_view_count fields in media_items_live_stream and media_items_video need moving to fake entries in the playback_history table. Would make sense just to create entries with session_id set to null and original_session_id set to 0000000000000000000000000000000000000000, where each entry has constitutes_view as 1.

[tjenkinson]

https://github.com/LA1TV/Website/tree/feat-optimize-most-popular

[tjenkinson]

#592
#593