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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[dlmsr]

Hi there, I already had a quick look: The HTML emails look fancy but I am not so sure whether we really want to send HTML emails. Ideally, the look and feel of the emails should then match the Moodle theme otherwise it would be hard to reuse this plugin on other instances (like examweb).

I do like the new plain-text version, though.

However, I want to have a more detailed look at the code refactorings, maybe this is something we can merge in any case (together with the new plain-text versions).