[6.x] Ability to use custom components & hide "Run action" button in action modals

[duncanmcclean]

This pull request allows action developers to render custom Vue components instead of our default confirmation text. It also allows developers to hide the "Run action" button in action modals.

Use cases

• One use case for this is @ryanmitchell's cache tracker addon, where he wants to show a list of cache keys for a certain entry.
  • The custom component is used to list cache keys, and the "Run action" button is hidden because there's no real "action" behind it.
  • In this scenario, actions is the way of getting a dropdown item & modal for free on listings.
• Another use case for this might be refactoring the fieldset-deleter component.
  • Currently, it's not action because we display a list of places the fieldset is being used. All imports need to be replaced before the developer can delete the fieldset.