Postscreen view jingles: Allow user to disable; allow content creators to replace

[Nami Verasche]

Some users would prefer not to have these. For these users, there should be an option to turn them off.

Some content creators have also shown an interest in being able to customize them.

[Nami Verasche]

Commit 92f54c7 adds option to disable these. (Merged into backwards-compatible in commit 743d4ab) Commit 31e8175 allows customization player-side; Flexi-side support is still needed. (Merged into backwards-compatible in commit 28f6105)