Allow user to overwrite config using `$STORAGE_ROOT/mailinabox.conf`

[bilogic]

Based on comments from https://github.com/mail-in-a-box/mailinabox/pull/2349#issuecomment-1987193731, this PR attempts to provide a convention on how mailinbox determines its configuration

1. mailinbox comes with defaults stored in /etc/mailinabox.conf
2. Everything in 1 can be overwritten by defining it again in $STORAGE_ROOT/mailinabox.conf
3. Backwards compatible If $STORAGE_ROOT/mailinabox.conf does not exists

To make it easier for the reviewer(s),

• All code changes are found in https://github.com/mail-in-a-box/mailinabox/pull/2382/commits/a7beb721680d405f86567ba001caecdb5eb2fe7e, which tries to read from $STORAGE_ROOT/mailinabox.conf into DEFAULT_xxx

I hope this can be accepted so that I can further proceed with introducing a configurable TTL and DKIM_SELECTOR

#2352 #2383