Replace --persist flag with REPLACE_PLAYLIST .env, Remove stale persist toggles

[dammitjeff]

PERSIST was incredibly confusing for users and devs to understand whats going on, and made it difficult to debug features since true meant "don't delete" false meant "delete and replace." The UI already had to double-invert just for it to make sense for end-users. Not good lol.

Swapped out the --persist flag with a new env, and wired it up to the settings.jsx page.

REPLACE_PLAYLIST=true - Delete old playlist, Create new one (this is now default)
REPLACE_PLAYLIST=false - Keep old playlist, Create new one alongside old playlist

Also added a one time migration in server.go to help convert existing "persist" values for anyone that was already using it, on startup. Will delete at a later date once people are migrated over.

Also removed stale --persist toggles in the frontend as they're now redundant with the toggles in settings.

[dammitjeff]

Hey! Made a few updates now that the big refactor is completed :)

I removed the env and switched back to a --replace_playlistflag. The default is now to overwrite the playlist unless the run has --replace-playlist=false to better reflect users behavior.

The frontend his now also wired up in place now, and keeping it as a flag means now it can be toggled on a per-playlist basis!

A description of what the toggle does appears on hover of the info icon. :)

Also removed the old "Update playlist in place" setting in the settings page as it's now redundant.

I'm ready for merge now to upstream but, definitely let me know if you have questions! I tested this pretty thoroughly and now it works REALLY nice. From the user's end, it finally feels like an actual self hosted auto updated playlist.

[LumePart]

Thanks for thew PR :)