Refine display of settings button

Ensure the Gap settings button can only ever appear when the user editor preference is set to either Tiny or Atto.

CSS Linting

Linting CSS including ensuring dropdowns don't have trailing CR/LF equivalent.

TinyMCE Support in per gap feedback

Support tinyMCE when adding per gap feedback

Moodle 5.1 Compatibility

Confirming Moodle 5.1 compatibility and fixes for standards compliance issues shown by phpcs

Moodle 5.0 compatibility

Moodle 5.0 compatibility and code to ensure restoration of backups does not generate duplicate questions

Moodle 4.5 and PHP 8.3

Moodle 4.5 compatibility. Confirmed PHP 8.3 compatibility.

v2.140

Version bump for deployment purposes

v2.139

PHP 8.2 and Moodle 4.4 compatibility

PHP 8.1 deprecation and letterhints on upgrade

Fix for PHP 8.1 deprecation messages.
Re-instate the creation of letterhints field in the db for upgrades from before v1.97 (in 2018).

Darktheme on mobile and long answers

Fix compatibility on mobile with dark theme (css tweak) #100

Centre droptarget text to better handle long answers #99

Additional English language sample questions.