Fix repeaters are registered without a populated item

[Tofandel]

Repeaters are registered in the Controller constructor without any model (I'm guessing so that the repeaters are available to the TwillBlocks facade early on)

But this is problematic as then if you change available select options or fields based on the model, the repeater templates are not updated, this PR fixes this issue by registering the repeaters again when they have been populated

It also fixes a performance issue with ->form() being unnecessarily called twice everywhere in the controller (the form method already shares form to laravel)