This is an extension of #427. Building on that change it adds support in the
aliases table for flagging aliases as:
 1. Applicable to inbound and outbound mail.
 2. Applicable to inbound mail only.
 3. Applicable to outbound mail only.
 4. Disabled.

The aliases UI is also updated to allow administrators to set the direction of
each alias.

Using this extra information, the sqlite queries executed by Postfix are
updated so only the relevant alias types are checked.

The goal and result of this change is that outbound-only catch-all aliases can
now be defined (in fact catch-all aliases of any type can be defined).

This allow us to continue supporting relaying as described at
https://mailinabox.email/advanced-configuration.html#relay
without requiring that administrators either create regular aliases for each
outbound *relay* address, or that they create a catch-all alias and then face a
flood of spam.

I have tested the code as it is in this commit and fixed every issue I found,
so in that regard the change is complete. However I see room for improvement
in terms of updating terminology to make the UI etc. easier to understand.
I'll make those changes as subsequent commits so that this tested checkpoint is
not lost, but also so they can be rejected independently of the actual change
if not wanted.