[Infrastructure] Provide a consistent editing experience between all 3 sites

Currently, the editing experience between the B.org, API and Forum sites is different:

B.org

API

Forum

B.org and the Forum at least have the same editor (CKEditor from core), but the button layout is different and the Format and Styles dropdown options are very different.
The API site uses a different editor altogether (BUEditor).

I propose updating the API site to use core's CKEditor, and then standardising the following between all three sites:

• The available buttons
• The button layout/ordering/grouping
• The options for the Format and Styles dropdowns

As discussed in backdrop-ops/docs.backdropcms.org#24, it seems BUEditor is preferred when writing lots of HTML code and so will likely stay on the API site. We should therefore install it as an option on other sites too for consistency. Then editors (people) can choose whichever editor (UI) they're most comfortable with.

[jenlampton]

I'm not sure having a consistent editing experience on different sites is important. I manage more than 60 sites in my day-to-day work, and none of those sites have exactly the same editing experience. Every site has the best editing experience for it's own needs.

I think it's more important that the forum have the best editing experience for a forum, and the API site have the best editing experience for creating documentation. We don't need @ mentions on the API site, and we probably don't need BU editor on the forum either.

I'd like to avoid adding the extra work of maintaining unnecessary features. I'd prefer to keep things simple and do the least amount possible to create the best experience for what is needed.