[epic] Remove Extension API and controllers from main branch (until after v1 release)

[joelanford]

The intent is to remove this API and its controllers for the v1.0.0 release. Once we've released 1.0.0, we plan to re-introduce the Extension API and controllers.

[acmenezes]

/assign

[everettraven]

@joelanford This epic is marked as "Needs Docs". Is this still the case or can this be closed out?

[joelanford]

Not sure! We just need to comb through docs and make sure we don't have anything in there about the old Extension API.