Compile CSS custom properties to hex values in mailers stylesheets

What does this MR do and why?

Compile CSS custom properties to hex values in mailers stylesheets

• Add PostCSS plugins
• Move mailer stylesheets to mailers/ directory
• Update import paths
• Remove asset precompile rules, use wildcard
• Add mailers processor to CSS compile script

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots or screen recordings

What	After
/rails/mailers/notify/pipeline_failed_email
/rails/mailers/license_mailer/approaching_active_user_count_limit.html

How to set up and validate locally

Validate CSS

1. Run yarn build:css --watch
2. Have a look at e.g. app/assets/builds/mailers/notify_enhanced.css. A lot of custom properties should now be inlined (if not, all)

Validate email styles

1. Remove pre-compiled CSS assets: rm app/assets/builds/**/*.css
2. Navigate to the pages above
3. Write a comment which mentiones a few labels.
4. Go to /rails/letter_opener. Wait until the notification email "comes through"

changed milestone to %17.2

added Category:Design System dark mode devopsmanage emails foundationdesign tokens groupdesign system sectioncore platform workflowstart labels

assigned to @sdejonge

removed dark mode label

removed workflowstart label

@leipert could you please review? Thanks!

requested review from @leipert

added pipelinetier-1 label

Proper labels assigned to this merge request. Please ignore me.

@sdejonge - please see the following guidance and update this merge request.

	1 Error
	Please add typebug typefeature, or typemaintenance label to this merge request.

mentioned in merge request !158838 (merged)

added backend frontend labels

	2 Warnings
	The master pipeline status page reported failures in rspec-ee unit pg14 24/28 rspec-ee unit pg14 9/28 rspec-ee unit pg14 8/28 If these jobs fail in your merge request with the same errors, then they are not caused by your changes. Please check for any on-going incidents in the incident issue tracker or in the #master-broken Slack channel.
	You've made some app changes, but didn't add any tests. That's OK as long as you're refactoring existing code, but please consider adding any of the maintenancepipelines, maintenancerefactor, maintenanceworkflow, documentation, QA labels.

	1 Message
	CHANGELOG missing: If this merge request needs a changelog entry, add the Changelog trailer to the commit message you want to add to the changelog. If this merge request doesn't need a CHANGELOG entry, feel free to ignore this message.

Reviewer roulette

Category	Reviewer	Maintainer
backend	@arpitgogia (UTC+5.5, 4.5 hours behind author)	@kerrizor (UTC-7, 17 hours behind author)
frontend	@rcrespo3 (UTC-4, 14 hours behind author)	@kushalpandya (UTC-4, 14 hours behind author)
~"Tooling"	Reviewer review is optional for ~"Tooling"	@psjakubowska (UTC+2, 8 hours behind author)

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Generated by Danger

added typemaintenance label

resolved all threads

mentioned in merge request !158909 (merged)