Forms: Update feedback email template

[enejb]

Resolves FORMS-40

This PR updates the email template that is send to admins when someone submits the form.

We are doing this so that the email that goes out look a more modern. The current email template does not apply any styles so the emails. So fonts and such look dated.

This PR is a revert or a revert ( + bug fixes )
This version of the email template doesn't include the subject line in the email copy so that it doesn't show up twice in some email clients.

Before:

After:

Proposed changes:

• Update the design to look more modern.
• Add a footer linking to jetpack forms.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

No

Testing instructions:

On a jurassic.ninja site or a hosted jetpack site. Apply this branch.

• Create a form. Insert it into a page or post.
• Fill out the form.
• Check the email that get sent to the admin across different email client. Mobile and desktop.
• Notice that it looks good across them.

[github-actions]

Thank you for your PR!

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• ✅ Include a description of your PR changes.
  
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• 🔴 Add a "[Type]" label (Bug, Enhancement, Janitorial, Task).
  
• ✅ Add testing instructions.
  
• ✅ Specify whether this PR includes any changes to data or privacy.
  
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Follow this PR Review Process:

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[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WoA dev site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin (Jetpack), and enable the update/feedback-emails-take-2 branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack update/feedback-emails-take-2

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[jp-launch-control]

Code Coverage Summary

Coverage changed in 2 files.

File	Coverage	Δ%	Δ Uncovered
projects/packages/forms/src/contact-form/templates/email-response.php	0/9 (0.00%)	0.00%	7 💔
projects/packages/forms/src/contact-form/class-contact-form.php	556/861 (64.58%)	-0.04%	0 💚

Full summary · PHP report · JS report

_{Coverage check overridden by I don't care about code coverage for this PR Use this label to ignore the check for insufficient code coveage. .}

[simison]

Rebased for testing

[simison]

Tests well, the double subject in previous (reverted) version, which in some email clients would indeed end up right above each other:

New version in this PR:

Trunk without this change:

[simison]

Rebased, @enejb I think this is ready to go?