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)
I think with the addition of the `closePassive()` above the call to save that it probably wasn't needed anymore in `showErrors()`, so :+1: