Simplify locations

[bartfeenstra]

Can we simplify locations by requiring paths to be absolute? Then we can convert all other paths, if they are relative, to absolute ones in relation to these location paths.

A use case is adding file patterns (for rsync's --exclude= and --include=). rsync does not seem to resolve those against the source path if the patterns are relative.

The implementation would involve moving the paths from locations to the top-level configuration. We can simplify the configuration files' source and target settings by repurposing those to just take strings paths. A newly introduced source_type and target_type would default to path, and a newly introduced source_configuration and target_configuration would contain any optional location configuration.

For local path locations, the source and target values may be relative. They will be provided in absolute form by resolving them against the location of the configuration file being used.

HOWEVER, this all depends on the source and target paths not changing. I don't foresee any problems within the scope of the use cases envisioned for Backuppy, but it's something to keep in mind.