[FR] Allow Reuseable Content Block Fields

[theAdhocracy]

The Content Block Fields have been a really handy way of standardising groups of fields between Entry types, but being able to reuse them multiple times within a specific Entry type (as with other fields) would make them even more powerful.

For instance, a pattern we've run into a couple of times:

• Create a set of Heading (text field), Content (CKEditor field), and isEnabled (lightswitch) toggle for use with announcement banners, all contained in a Content Block called "Announcements"
• That "Announcements" field is then added to a number of Entry templates – homepage, dashboard, inbox etc. – so that you can define different banners for different pages
• But then we want to repeat that process for e.g. email templates. We have a "Global Set" (now Entrified) that controls the text in emails, with each email handled by a specific Tab
• We want each tab to have an Announcement, but we can only set it in one place, so we end up recreating the layout for each tab instead

Then, if we ever need to change that global pattern, we have to remember to modify the email templates differently, which undercuts the utility of the Content Block in the first place.

(Ideally, I'd love for all fields to be fully reuseable/duplicatable, but this just has a concrete use-case we've run into a few times)