Use proposed workflow for custom email settings page

The following discussion from !128637 (merged) should be addressed: !128637 (comment 1509191997)

There is currently an effort to migrate settings page cards to use a consistent pattern. Here's the meta issue with more details #419105 (closed) (cc: @annabeldunstone @seggenberger @jmiocene )

It might make sense here to use this pattern from the beginning. Doing so also helps answer which buttons make sense.

Here is a super rough version of how that could look. I've made it look really draft to make it clear I don't have enough context to have full confidence. Its not a proposal, but hopefully a useful idea.

Hope it helps!