sourcedr: Extract Project class from server.py

This commit adds an abstraction for Project so that the review project
files can be used by other command line tools.

This commit also removes several Flask global variables.  Now, the
functional tests will only rely on the `create_app()` factory function.

Test: ./sourcedr/functional_tests.py
Change-Id: Id285664bb4d7c5788c50fd9edb12cb691ae9b9f6