Feature: course filter settings in databse

[TamaroWalter]

Note: Please fill out all relevant sections and remove irrelevant ones.

🔀 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:

Until now only the settings of the letter and time filter were saved in the database. After a little bit of thinking, the course filter settings can be stored too. Until now the problem was that the course filter has a dynamic amount of courses. Courses may change (new courses appear, old courses are removed).

Instead of saving a value for each course or opening new tables that matches a course to a userid (which is the clean relational-way), a trick is used. A new attribute in the table saves a stringified version of a JSON object that stores the setting of each course that shows content in townsquare with courseid => true/false.

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📋 Checklist

Please confirm the following (check all that apply):

• I have phpunit and/or behat tests that cover my changes or additions.
• 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.
• If there are changes in the database: I updated/created the necessary upgrade steps in db/upgrade.php and
  updated the version.php.
• If there are changes in javascript: I build new .min files with the grunt amd command.