mu: allow option to set muhome

[NovaViper]

Description

Supersedes and closes #5535
Allows for the database path for mu to be configured. Useful for keeping the maildir and mu xapian cache together without having to modify XDG_CACHE_HOME. Add test to check for custom home setting.
Fixes #5534

Checklist

• Change is backwards compatible.

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Maintainer CC

@Karljo

[rycee]

Thanks! Merged to master now 🙂