make the format of the usage event match what Central is expecting

[annthurium]

The metrics format is documented here: https://github.com/github/client-apps/pull/71/files

This pull request:

• separates the usage data into dimensions and measures
• adds lokijs as a store. localStorage is nice but it doesn't support objects or arrays as values, and we want to be able to get all the measures without looping through all the values in localStorage. lokijs is actively being developed, desktop wants to migrate to it, and it's similar in size to Dexie but with (imo) a friendlier api.
• adds the date to the metrics, formatted to iso standard whatever whatever

[smashwilson]

Pay no attention to the failing checks -- they should be resolved as soon as #4 is merged, then merged in here :eyes: