Add test coverage for meeting attendance endpoints

@DSASanFrancisco/portal-members

Adds unit tests covering all of the meeting attendance endpoints in members.py. This patch increases test coverage of members.py from 42% to 69%. I fixed a couple minor issues discovered while writing the tests:

• /meeting/attend was returning a 400 instead of a 404 for non-existent meetings
• /meetings/<id>/attend was returning a 400 when attendance was entered twice for the same member

The TestWebMembers suite now recreates the contents of the database for each test so that tests can verify mutations (like adding a meeting attendance) without interfering with the expectations of other tests.