R1:442590348014

Don't require phabricator.base-uri to be configured

Summary:
Fixes T2293.

We currently hard-require this in setup. We do not need to; we don't actually need it until we start running daemons. Move it to post-install and provide more guidance.

We could make this even easier in the future, but we'd need to special case it, since it's dangerous to let it be set to any value (if you set it to the wrong value, you can't log in). We could safely have a workflow which writes the current request URI into the…