Feature/metronome-bank-interval

This pull request modifies the metronome model and view so that the admin can specify that a chord "bank" occurs at some arbitrary interval after the metronome ticks. They should be able to specify this as a fraction of the metronome interval in a config file. The motivation for this was that the chord banking was occurring right on the metronome beat, but in practice, that was a little too soon. The user would prefer to have it bank slightly after the beat and be able to configure that delay depending on what was being played or practiced.

The following changes were made:
- Added a "tock" event to the metronome. The idea is that the metronome will always trigger a "tick" followed by a "tock." The exact time at which a "tock" occurs after a "tick" is determined by a config value expressed as a fraction of the tick interval.
- Modified the metronome view to listen for "tock" events and trigger a "bank."
- Added a new config value that can be edited.

@jazahn can you please review this?