Updated Markdown plugin to use the PHP Markdown Extra library.
Added the `get_slugs()` method to the `CustomField` model.
Added the `ticket_filters_for($project)` helper.
Started adding support for custom fields to the ticket filter SQL query helper.
Removing of a ticket filter is now done via JavaScript and does not auto-submit the form.
All ticket filters can now be removed.
Changed the ticket filter help functions to return both the URL string and name of the filter.
Add copyright and package to the `ticket_filters_for()` helper.
Custom field filters working.
The ticket filters form visibility can now be toggled.
Merge remote-tracking branch 'main/master'