Bug 29330: Restore handling of serialized MIME messages in message_queue

Bug 29330: Restore handling of serialized MIME messages in message_queue

This patch changes how multipart MIME messages are handled on the
message_queue table.

The email, with the required attachments, is now generated using
Koha::Email, and serialized using Koha::Email->as_string.

This bug also adds Koha::Email->new_from_string which is used to read
that data from the DB, and produce a Koha::Email object, that can be
further augmented/modified using regular Koha::Email methods.

This implementation should be considered a middle ground, with
backportability in mind. higher-level methods should encapsulate setting
the default headers and addresses, to clean the area a bit further.

Preparation:
- You need a valid SMTP configuration in koha-conf.xml. If you use Gmail
  you can generate an 'app password' and set things like this:

 <smtp_server>
    <host>smtp.gmail.com</host>
    <port>587</port>
    <timeout>5</timeout>
    <ssl_mode>STARTTLS</ssl_mode>
    <user_name>youraddress@gmail.com</user_name>
    <password>youpassword</password>
    <debug>1</debug>
 </smtp_server>
- Set KohaAdminAddress to your address.

To test:
1. Pick a patron. Make sure it doesn't have any email address (Acevedo?)
2. Set an overdue notice trigger for its category
3. Check something out, with due date in the past to force an overdue
4. Run:
   $ kshell
  k$ misc/cronjobs/overdue_notices.pl -v
  k$ exit
   $ koha-mysql kohadev
   > SELECT * FROM message_queue WHERE borrowernumber=the_borrowernumber;
=> SUCCESS: A notice has been created
5. Run:
   $ kshell
  k$ misc/cronjobs/process_message_queue.pl --verbose
=> SUCCESS: SMTP is ok => Email is sent
=> FAIL: Your inbox shows an email with weird content
6. Apply this patches
7. Run:
   $ koha-mysql kohadev
   > DELETE FROM message_queue;
8. Repeat 4 and 5
=> SUCCESS: You got an email with an attachment!
=> SUCCESS: The attachment contains an email that couldn't be delivered!
9. Try all the things that enqueue messages :-D
=> SUCCESS: No behavior change
10. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Edited the POD, restoring a few lines that describe the needed hash
keys of the attachments.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit 54546da3f07380375470bc130f15a33830419ac8)
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>