Moar modal fixes

• Align the header title in the modal with the closing X symbol
• I didn't want the fact that a particular action uses a modal to leak in the UI store. Unfortunately, this seemed to be unavoidable in the case of entity's deletion: if we emit the action to hide the modal after the reducer is done, then the modal will try to re-render with an entity that doesn't exist anymore. I think I've got a solution for this, using an augmented action that would hide the modal on success.
• maybe the button to show a modal could be made generic.
• maybe there's a better way to encapsulate state construction for a modale. In addition to the slug, a modal could export a "createModalState" function that could be called with named parameters.

I'll try to fix this.