user invites: Ability to invite users post-signup on enterprise instances

Created by: slimsag

Spawned from this doc and after discussions with Rob, Philipp, and Cloud team.

We want a way to enable users to email-invite others who are not yet on Sourcegraph for enterprise instances. This should be possible post-signup (and we probably shouldn't care about the sign up flow at all), because e.g. with SSO configured there is no "sign up" flow on-prem.

Dependencies:

• https://github.com/sourcegraph/sourcegraph/issues/31215
• https://github.com/sourcegraph/sourcegraph/issues/31216
• #31702 (closed)
• #31558 (closed)
• #31777 (closed)
• #31703 (closed)
• #31778 (closed)
• #31780 (closed)