[Feature]: Sync the same path to multiple services

[bear454]

Feature Description

I have a primary and backup sync services; both use the same file structure.

Celeste is currently enforcing uniqueness of the local and remote paths but is not accounting for those paths being on different services.

Please extend the sync_dirs entity/database to consider uniqueness to include the remote, so that, for example ~/Documents can be synced to Documents on multiple services.

What version of Celeste are you using?

v0.8.3