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 database configuration, or a two-stage workflow where we set the URI and then verify it, but these both imply some special casing and complication. This should be a step forward from where we are today, regardless.

Test Plan:
Removed "phabricator.base-uri" from my configuration. Verified Phabricator still works.

Without "phabricator.base-uri" configured, logged in from multiple host names (127.0.0.1:8080, local.aphront.com:8080).

Configured "phabricator.base-uri". Verified my unblessed session no longer worked. Verified setup issue went away.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2293

Differential Revision: https://secure.phabricator.com/D4580