refactor notification system to produce only the minimum number of notifications

[falquaddoomi]

In the current implementation, in addition to variant-specific notifications all users will receive a "pre-notification" that causes them to open the app, triggering the variant-specific notifications to be processed and possibly shown. In the common case most users won't be subscribed to variants that have changed in a given update, so they'll receive a spurious notification and be taken to an empty list of relevant changes.

The "bug" here is to remove this spurious notification, yet maintain the privacy of user's variant subscriptions.

[falquaddoomi]

Plan to fix the above:

• create a background task that checks for database updates
• should it find an update, the task downloads the full diff for the latest version (which is the same as what we're doing with the push notifications currently)
• the task then filters the diff down to the locally-stored followed variants
• if there are more than one variant, the app raises a single "x variants' pathogenicities have changed" notification. Clicking this notification displays the "Notify Log" screen, which lists the changed variants.
• if there is a single variant, the app raises a notification specific to that variant which mentions the new pathogenicity in the message. Clicking this notification displays the details view for that variant.

[falquaddoomi]

Work on this was started in 4ac573eae00032952261b31797d4db60ff6189eb and continued until 24ca8c22509d61a34bfbca71e4181bf306d9cdbb as of this writing. It seems to be implemented correctly, but it needs to be tested.

Caveat(s):

• if the user subscribes to variants that happen to be in the most recent version before the task runs, they'll receive notifications that those variants have changed. this almost surely won't be a problem because the first iteration of the background task will run nearly as soon as they first open the app the first time, but it's worth testing.
• users who have their phone off for an extended period of time (e.g. more than a month) will receive notifications for only the most recent version. we should discuss if we want to announce every intermediate version as well.