immich upload and library mounts should be combined

[NicholasFlamy]

https://immich.app/docs/guides/custom-locations/

The upload and library mounts should be combined to prevent excessive wear.

Originally posted by @NicholasFlamy in #411 (comment)

[NicholasFlamy]

I have a suggestion to add, By default have the mounts unchecked and go to default best practices of being part of the library mount and if someone wants to specify they can hit the checkbox to show the mount option, this would also work for migrations.

I could PR this, but I've never done the migrations part.