In some very rare cases CssParser could be loaded but CssParser::Parser not
this ensures we check for the actual constant we plan to call for concurrent
digest generations
This feature adds the ability to customize the HTML part of all emails using a custom HTML template and optionally some CSS to style it. The CSS will be parsed and converted into inline styles because CSS is poorly supported by email clients. When writing the custom HTML and CSS, be aware of what email clients support. Keep customizations very simple.
Customizations can be added and edited in Admin > Customize > Email Style.
Since the summary email is already heavily styled, there is a setting to disable custom styles for summary emails called "apply custom styles to digest" found in Admin > Settings > Email.
As part of this work, RTL locales are now rendered correctly for all emails.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
* Rearrange frontend to account for mailing list mode
* Allow update of user preference for mailing list frequency
* Add mailing list frequency estimate
* Simplify frequency estimate; disable activity summary for mailing list mode
* Remove combined updates
* Add specs for enqueue mailing list mode job
* Write mailing list method for mailer
* Fix linting error
* Account for stale topics
* Add translations for default mailing list setting
* One query for mailing list topics
* Fix failing spec
* WIP
* Flesh out html template
* First pass at text-based mailing list summary
* Add user avatar
* Properly format posts for mailing list
* Move make_all_links_absolute into Email::Styles
* Apply first_seen_at to user
* Send mailing list email summary hourly based on first_seen_at
* Branch and test cleanup
* Use existing mailing list mode estimate
* Fix failing specs
