Prior to this commit, the composer was based on a map[string]string.
While this approach was very versatile, it lead to a constant encoding / decoding
of addresses and other headers.

This commit switches to a different model, where the composer is based on a header.
Commands which want to interact with it can simply set some defaults they would
like to have. Users can overwrite them however they like.

In order to get access to the functions generating / getting the msgid go-message
was upgraded.

We made a new type out of go-message/mail.Address without any real reason.
This suddenly made it necessary to convert from one to the other without actually
having any benefit whatsoever.
This commit gets rid of the additional type

Go pre 1.15 parsed an empty string as an error, 1.15 did not but this was
unintentional as per https://github.com/golang/go/issues/40803

Fix the comment accordingly

Go 1.15 handles "" in the address parser as a non error case, returning
an empty list.
Prior versions returned an error, which is not what we want.

Reported-by: anianz <a.ziegler@cioplenu.de>
Tested-by: anianz <a.ziegler@cioplenu.de>

Not sure we need a flag for it, most people expect times to be in their local
time anyhow.

This allows us to hook into the std libs implementation of parsing related stuff.
For this, we need to get rid of the distinction between a mailbox and a host
to just a single "address" field.

However this is already the common case. All but one users immediately
concatenated the mbox/domain to a single address.

So this in effects makes it simpler for most cases and we simply do the
transformation in the special case.

If a message date would fail to parse, the worker would never receive
the MessageInfo it asked for, and so it wouldn't display the message.

The problem is the spec for date formats is too lax, so trying to ensure
we can parse every weird date format out there is not a strategy we want
to pursue. On the other hand, preventing the user from reading and
working with a message due to the error format is also not a solution.

The maildir and notmuch workers will now fallback to the internal date, which
is based on the received header if we can't parse the format of the Date header.

The UI will also fallback to the received header whenever the date header can't
be parsed.

This patch is based on the work done by Lyudmil Angelov <lyudmilangelov@gmail.com>
But tries to handle a parsing error a bit more gracefully instead of just returning
the zero date.