Diffusion + Herald - warn users if importing repository

Summary: 'cuz things fail a bunch until importing is done. Fixes T4094.

Test Plan: set isImporting to return true. Browsed Diffusion and saw helpful warnings everywhere. Browse Herald transcript and saw a helpful warning

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4094

Differential Revision: https://secure.phabricator.com/D8903