The policy here is essentially:
* Pages on the central server (documentation, etc.) are server_uri.
* Pages associated with a user's realm server (anything logged-in, plus
things like their login page) are realm_uri.
Helps in case the development environment is not using https.
We get the value from zerver/context_processors.py (which is
ultimately reading it from settings)
The queued email gets deleted if the user signs up before it gets sent.
Otherwise, they are reminded in 2 days that they still haven't signed up.
This addressses Trac #1812
(imported from commit c1bdc09c03ac576b08986e56994de72d52fd293b)