Refactor communications

[sasha0]

Refs #319 #783

Summary:

• Created separate app - communication.
• Dispatcher moved to communication app's utils.
• Dispatcher improved with methods to send each separate email.
• Added ability to send email with attachments.
• All event codes (for each separate email) now kept in Dispatcher.
• Sending notifications moved to communication app as well.
• Added ability to chose if sent email should be saved to DB (through OSCAR_SAVE_SENT_EMAILS_TO_DB, True by default).
• All relevant models moved from customer app to communication app (with data migrations) - CommunicationEventType, Email and Notification.
• Moved classes and models added to MOVED_CLASSES and MOVED_MODELS.
• Tests adapted to check new functionality.
• Added tests to check the sending of each separate email.
• Deleted TestOrderPlacementMixin - Now we use Dispatcher to send emails and code (event code) does not used when we get message context for email that informs about a placed order.
• All emails' templates (commtype_*) moved from customer/emails to communication/emails.
• Templates from customer/email/*.html and customer/notification/*.html moved to communication/email/*.html and communication/notification/*.html.

[codecov]

Codecov Report

Merging #2591 into master will decrease coverage by 0.08%.
The diff coverage is 86.19%.

@@            Coverage Diff             @@
##           master    #2591      +/-   ##
==========================================
- Coverage   84.62%   84.54%   -0.09%     
==========================================
  Files         279      286       +7     
  Lines       15045    15135      +90     
==========================================
+ Hits        12732    12796      +64     
- Misses       2313     2339      +26

Impacted Files	Coverage Δ
src/oscar/apps/communication/managers.py	100% <ø> (ø)
src/oscar/__init__.py	33.33% <ø> (ø)	⬆️
src/oscar/apps/customer/models.py	100% <ø> (ø)	⬆️
src/oscar/apps/customer/abstract_models.py	80.5% <ø> (-6.07%)	⬇️
src/oscar/apps/order/abstract_models.py	93.2% <ø> (ø)	⬆️
...anagement/commands/oscar_generate_email_content.py	0% <0%> (ø)	⬆️
src/oscar/management/commands/oscar_send_alerts.py	0% <0%> (ø)	⬆️
src/oscar/apps/communication/config.py	0% <0%> (ø)
src/oscar/apps/communication/app.py	0% <0%> (ø)
src/oscar/apps/checkout/mixins.py	89.85% <100%> (+0.24%)	⬆️
... and 32 more

[craigloftus]

@samitnuk @sasha0 To help guide the review would you be able to provide a summary of the changes in the PR, what the intention in the PR is and whether there is ambition for a follow up PR to refactor further?

[samitnuk]

@solarissmoke Many thanks!

This PR updated based on your comments. Please review when you will have a chance.

[solarissmoke]

Thanks @samitnuk. I think this is ready and will rebase and merge in #3260.