People: Create invite accept/decline notices

[v18]

At the end of the accept invite flow, we'll need to provide a UI that lets the user know that he/she successfully accepted the invite.

These are notices, not a new component.

@rickybanister mentioned that it's worth searching the codebase for Welcome. He took that design from the welcome message shown after signing up for a new account. Something must already exist that spits it out.