Add support for building with Bazel
Description
To improve the usability of BuildGrid, add the infrastructure necessary to build and run it with bazel.
The tests are not very rigorous: it only does a basic smoke test of the app being able to start (which indicates, at least, that all the imports work). Currently it would be bit of a hard-to-maintain change to change all the tests to support running with bazel. Waiting rather for https://github.com/bazelbuild/rules_python/pull/243 to be done.
The hardcoded list of requirements in the BuildGrid BUILD file is non-ideal, I don't believe there is a way to populate that dynamically from requirements.txt.