DB-query integration tests

[wjordan]

This PR adds a few integration tests on the number of ActiveRecord DB queries generated by specific route actions. The implementation in the CaptureQueries module uses the ActiveSupport::Notifications subscription API to capture all ActiveRecord queries generated within a block, and assert an expected count.

This will allow us to more easily track (and automatically prevent regressions on) the number of database queries performed by high-traffic routes.

I've added a few basic tests to get started:

• milestone POST (passing, signed-in user) expects 11 queries
• milestone POST (failing, signed-in user) expects 11 queries
• user_progress GET (signed-in user, with existing user_level) expects 7 queries
• script_levels #show (signed-in user with existing user_level) expects 22 (!) queries

These query counts are not ideal, and the numbers will be reduced in an optimization PR that will come after these integration tests.

I've refactored the existing CachingTest tests to use this implementation instead of calling no_database (which stubs ActiveRecord::Base#connection), since I think it's a cleaner approach.

This PR also renames an API route-path fragment from :script_name to :script- it turns out that :script_name is a reserved word in Rails path helper, and its use was preventing the new integration tests from working properly.

[jopolsky]

Looks good to me.