Sitewide account

For anyone following along, or for future me, my immediate use-case for this is that it is prerequisite for implementing the Social Platform plugin (social post controller module). Social Post Controller can not make use of the Toot action plugin, since rather than posting on save, social post controller instead queues the posts (so that posting can occur asynchronously). It may be worth considering whether that queue could be generated at some point also.

Anyway, the blocker is that a plugin for Social Post Controller does not currently have a way to determine which social account should be posted to. Implementing a site-wide user here is a quick, low barrier fix that could be generally useful.

In a meeting, we determined that we will actually move forward with the first alternative:
allow admin to authorize posting from the admin settings (rather than user context)

Tested adding and deleting accounts for both the sitewide and user context. All seems to be working okay.