Add a new UI for all emails

[TamaroWalter]

🔀 Purpose of this PR:

• Fixes a bug
• Updates for a new Moodle version
• Adds a new feature of functionality
• Improves or enhances existing features
• Refactoring: restructures code for better performance or maintainability
• Testing: add missing or improve existing tests
• Miscellaneous: code cleaning (without functional changes), documentation, configuration, ...

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📝 Description:

The emails were a mess, they were ugly and the creation used a lot of unreadable parameters ($params->a, $params->e). I redesigned the ui and process of sending a mail:

• Add html and text/plain emails for all 4 emails (course request, request accepted, request denied, category relocation wish)
• Use maizzle to create the html emails with a beautiful styling (screenshot can be seen at the end). The style for the html is based on the Learnweb colors to give a emails a more professional touch
• Make the email creation more readable: first set up a minimal amount of parameters (only the user id for example) for the ad-hoc task (email get send by an ad-hoc task), and then the task gathers all important data like user or course name.
• Delete the current unit tests. They only tested if the emails were send but that is logic that is not covered in the lsf_unification domain but in the moodle core. Our tests should cover our functions.

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📋 Checklist

Please confirm the following (check all that apply):

• Code passes the code checker without errors and warnings.
• Code passes the moodle-ci/cd pipeline on all supported Moodle versions or the ones the plugin supports.
• Code does not have var_dump() or var_export or any other debugging statements (or commented out code) that
  should not appear on the productive branch.
• Code only uses language strings instead of hard-coded strings.

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🧾📸🌐 Additional Information (like screenshots, documentation, links, etc.)

How the email looked before:

How the mails look now:

Html	text/plain

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