Forms Template/JS Refactoring

[felixrindt]

I intend to merge this with rebase (no squash).

• refactor PluginForms to use form templates instead of FormHelper
• update jQuery to 3.7.0
• reduce flicker in formset handler by
  • hiding the new form before it is added to the dom when using animation, therefore no flicker when clicking the add button
  • creating a new event type to handleForms before the slideDown animation. Before, the animation would compute before handleForms was called and therefore use the wrong html (e.g. the vanilla select-multiple instead of select2 which is smaller) leading to jumps at the end of the animation

[coveralls]

coverage: 88.077% (-0.2%) from 88.251% when pulling 4b9f152 on refactor-plugin-forms into e8802ee on main.