POC for the composerisation of Mautic TPROD-147

[nickveenhof]

Description:

This PR moves the plugins out of the codebase and allows Mautic to be installed with different ways. This PR ideally doesn't change the way people can install mautic, but it offers possibilities such as

composer create-project mautic/recommended-project:4.x-dev mauticcomposerproject --no-interaction

What is still open before this can get green light

• the .htaccess files need to be added to vendor if it is installed from mautic/mautic instead of mautic/recommended-project. See also https://github.com/drupal/core-file-security/tree/9.2.x how it is done there.
• The themes are still in the main repository, this is fine for now, but it is blocking for the users of the recommeded-project part. Ideally the themes live in app/themes and themes are also allowed in themes/ or wherever custom themes are placed. This is different than plugins as plugins are always optional, themes are not.
• Many image files are not referenced correctly yet. The references still go to media/images, they should go to app/assets/images
• Update MauticInstaller to allow plugins & themes composer/installers#484 should be committed, then we remove the complex override of this repository and the file gets a lot simpler.

Steps to test this PR:

1. Load up this PR
2. See if Mautic still works as expected
3. Run composer update --with-dependencies, see if mautic still works as expecred

[codecov]

Codecov Report

Merging #9849 (61162df) into features (21cc28b) will increase coverage by 0.00%.
The diff coverage is n/a.

❗ Current head 61162df differs from pull request most recent head 7819dd5. Consider uploading reports for the commit 7819dd5 to get more accurate results

@@             Coverage Diff             @@
##             features    #9849   +/-   ##
===========================================
  Coverage       39.66%   39.67%           
- Complexity      33978    33979    +1     
===========================================
  Files            1989     1989           
  Lines          105731   105732    +1     
===========================================
+ Hits            41943    41949    +6     
+ Misses          63788    63783    -5     

Impacted Files	Coverage Δ	Complexity Δ
...dles/InstallBundle/Configurator/Step/CheckStep.php	7.44% <0.00%> (ø)	47.00% <0.00%> (+1.00%)
...serBundle/Security/SAML/Store/CredentialsStore.php	100.00% <0.00%> (ø)	12.00% <0.00%> (ø%)
...es/CampaignBundle/Executioner/EventExecutioner.php	78.69% <0.00%> (+2.95%)	49.00% <0.00%> (ø%)

[dennisameling]

@nickveenhof CS should be fixed now with 2c95643 - but we have a conflict due to a bunch of dependency updates from #9833. Would you mind having a look at that? Thanks! 🚀