Tag Meta Screen

Closes #4509, Closes #4615

- Adds meta title & meta description input fields
- Autosaves meta data
- Shows SEO preview (falling back to tag name & description if meta not supplied)
- Adds `type="button"` to delete button (which closes 4651)
Updated PR. I wasn't sure if you meant to remove the option on the `showErrors()` call, so it's gone. If that needs to be back, just shout. :)