Add categorizing admin UI to posts

[JSn1nj4]

Similar to #106: technical requirements exist, but no UI

• [x] Add a field for adding categories
• [x] If it does not exist (by slug), create
• [x] If it does exist (by slug), attach existing
• [x] Generate slugs from category name
• [x] Allow un-categorizing
• [x] Sync categories on save

Implementation

• [x] Livewire widget
• [x] Loading behavior
• [x] Checkbox values are form IDs (or slugs I suppose - done before with tags)
• [x] Checkbox list should load first 100
• [x] Preload checked state
• [x] "New Category" form
  • [x] hit server
  • [x] authorize
  • [x] sanetize
  • [x] validate
  • [x] create new category
  • [x] reload full list
  • [x] Clear "new" form state
  • [x] updating behavior (see "Loading" ☝️)
  • [x] Don't attempt attach if no categorizeable model present
  • [x] If category is already attached, skip
  • [x] If category exists in list, attach that one and update checked state
• [x] Shared input sanetizing - Categories widget, FormRequests

[JSn1nj4]

I believe attach() will just not attach a duplicate, so this should be fine.