Unify series thumbnails

[owi92]

With the recent addition of the "My series" table and the "Series details" page there are now three different places where the thumbnail stack for series is shown (the third one being in the search results). Each of these uses a different mechanism of prodiving these thumbnails and without a specific and enforced ordering we can't be sure that they will always match. One of these was previously ordered by the event creation date, ascending.

We should apply that to each instance (right now it's been disabled). Doing so is easy enough for the former two but a little more involved for the search results. This will need at least a new database migration and possibly an index rebuild.