Apply regular expression on richeditor to cleanup content

[didoda]

This provides a way to apply a regular expression on richeditor change, if set in configuration.
Example:

'Richeditor' => [
    'cleanup_regex' => '\s\w+="[^"]*"', // remove all attributes from tags
],

To investigate: should a "default regex" be used always, if no configuration set?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (3708ec2) to head (5345ce1).
Report is 20 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##              master     #1271   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity      1384      1391    +7     
===========================================
  Files             98        98           
  Lines           5493      5543   +50     
===========================================
+ Hits            5493      5543   +50     

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