community-contributions.py script should use Launchpad to determine who is not a Canonical employee

The utilities/community-contributions.py script, which updates https://dev.launchpad.net/Contributions automatically, currently contains a hardcoded list of Canonical employees known to hack on Launchpad, so they can be eliminated from the output (even when they commit from a non-Canonical email address, as often happens for historical reasons).

Instead, the script should use launchpadlib (https://help.launchpad.net/API/launchpadlib) to determine at run-time who is and isn't a Canonical employee. Ideally, it would even be able to figure out when that user joined the ~canonical team in Launchpad, so that contributions made before/after their employment would be counted as non-Canonical contributions.

This bug corresponds to the "XXX" comment above the 'known_canonical_devs' list in the code.

Setting Importance to "Low" because it's fairly easy to just update the script whenever we notice bogus inclusions on https://dev.launchpad.net/Contributions. It's just silly to keep doing that forever when there's a way to solve the problem permanently.