Email draft feature

[dadarya0]

Q	A
Bug fix? (use the a.b branch)	[ ]
New feature/enhancement? (use the a.x branch)	Y
Deprecations?	[ ]
BC breaks? (use the c.x branch)	N
Automated tests included?	Y
Related user documentation PR URL	mautic/mautic-documentation#...
Related developer documentation PR URL	mautic/developer-documentation#...
Issue(s) addressed	Fixes #...

Description:

Add 'email_draft_enabled' => true in local.php for enable email draft functionality.

Users would like to be able to edit a published email and be able to preview changes without them being published.

Workflow :

• A user has a published email and wants to make a change to it.
• The user will will create a draft from the email which is a clone of the email HTML content.
• The user will make the changes in the draft email HTML.
• The user may send the draft (webview preview) email to review to other person.
• When the change is approved the user can merge the change back to the original email or discard draft anytime.

Steps to test this PR:

1. Open this PR on Gitpod or pull down for testing locally (see docs on testing PRs here)
2. Add 'email_draft_enabled' => true in local.php for enable email draft functionality.
3. verify Icon on Email List page to show Draft if draft saved
4. User must be able to save their edits as a draft from the Edit Email screen using Save draft button
5. Verify Preview Draft URL must reflect draft state if it exists. verify email Preview should work as it is.
6. User must be able to push draft state to published state using Apply Draft button
7. User must be able to discard draft using discard draft button

[codecov]

Codecov Report

Attention: Patch coverage is 85.89744% with 33 lines in your changes are missing coverage. Please review.

Project coverage is 61.46%. Comparing base (714ba98) to head (ae209b0).

Additional details and impacted files

@@             Coverage Diff              @@
##                5.x   #13613      +/-   ##
============================================
+ Coverage     61.39%   61.46%   +0.07%     
- Complexity    34034    34122      +88     
============================================
  Files          2240     2244       +4     
  Lines        101714   101926     +212     
============================================
+ Hits          62443    62652     +209     
- Misses        39271    39274       +3     

Files	Coverage Δ
...undles/EmailBundle/Controller/PublicController.php	56.64% <100.00%> (+1.05%)	⬆️
app/bundles/EmailBundle/Entity/Email.php	91.28% <100.00%> (+0.98%)	⬆️
...bundles/EmailBundle/Event/EmailEditSubmitEvent.php	100.00% <100.00%> (ø)
app/bundles/EmailBundle/Form/Type/EmailType.php	98.08% <100.00%> (+0.26%)	⬆️
app/bundles/EmailBundle/Helper/EmailConfig.php	100.00% <100.00%> (ø)
app/bundles/EmailBundle/Model/EmailDraftModel.php	82.35% <82.35%> (ø)
...es/CoreBundle/Doctrine/AbstractMauticMigration.php	6.77% <0.00%> (-0.50%)	⬇️
...bundles/EmailBundle/Controller/EmailController.php	61.00% <91.83%> (+5.78%)	⬆️
...dles/EmailBundle/EventListener/EmailSubscriber.php	81.39% <85.10%> (+3.34%)	⬆️
app/bundles/EmailBundle/Entity/EmailDraft.php	68.75% <68.75%> (ø)

... and 1 file with indirect coverage changes

[RCheesley]

Awesome looking feature @dadarya0 - it looks like there's some conflicts to be resolved before we can test :)