Track when the verify button is pressed + robustness fixes

[shankari]

• Add the ClientStats module to infinite scroll
• Add a new kind of client stat
• Mark the stat when the button is pressed

Testing done:

• Hacked the code to ensure that verifiability is always set to "can-verify"
• Verified the trip
• "End trip and force sync"
• Confirmed that the value was pushed

2021-08-04 15:37:25,004:DEBUG:123145498861568:Updated result for user = 113aef67-400e-4e21-a29f-d04e50fc42ea, key = stats/client_nav_event, write_ts = 1628116460.869 = {'n': 1, 'nModified': 0, 'upserted': ObjectId('610b16a53906cf6a2964f11b'), 'ok': 1.0, 'updatedExisting': False}
2021-08-04 15:37:25,005:DEBUG:123145498861568:Updated result for user = 113aef67-400e-4e21-a29f-d04e50fc42ea, key = stats/client_nav_event, write_ts = 1628116560.485 = {'n': 1, 'nModified': 0, 'upserted': ObjectId('610b16a53906cf6a2964f11d'), 'ok': 1.0, 'updatedExisting': False}
2021-08-04 15:37:25,007:DEBUG:123145498861568:Updated result for user = 113aef67-400e-4e21-a29f-d04e50fc42ea, key = stats/client_time, write_ts = 1628116588.745 = {'n': 1, 'nModified': 0, 'upserted': ObjectId('610b16a53906cf6a2964f11f'), 'ok': 1.0, 'updatedExisting': False}

• Confirmed that the value was in the usercache at the end

edb.get_usercache_db().find({"metadata.key": "stats/client_nav_event"}).distinct("data.name")
['app_launched',
 'checked_diary',
 'checked_inf_scroll',
 'expanded_trip',
 'notification_open',
 'opened_app',
 'sync_launched',
 'verify_trip']

• two robustness fixes where we check for the inferred labels and the
  expectation before using them. These should not occur but it is good to avoid
  crashing if they do!

[GabrielKS]

Looks good!