Allow users to specify a custom DKIM selector

[bilogic]

• Continuation of custom DKIM selector #2349 which was closed due to my accidental deletion of the branch
• Also relies on storage-root-config Allow user to overwrite config using $STORAGE_ROOT/mailinabox.conf #2382

To make it easier for the reviewer(s),

1. auto format commits have no changes in logic
2. a7beb72 and before are for PR Allow user to overwrite config using $STORAGE_ROOT/mailinabox.conf #2382
3. DKIM selector code changes are found in only 1 commit 12f1486, which basically replaces the hardcoded selector with a variable

[JoshData]

Sorry I'm not going to allow putting the setting in mailinabox.conf. It has to go somewhere in STORAGE_ROOT so that it's backed up.

[bilogic]

Ok, how about STORAGE_ROOT/config.d/override.conf?

It would be great to get faster feedback. I had to read up what this was all about before being able to answer. :)