Fix PostgreSQL errors when a metric activity is recorded twice

[jaller94]

Every time a user posts in a room they have already posted in on that day, the bridge outputs an error, if the metrics are enabled:

Apr-16 11:24:55.729 ERROR BridgedRoom Error storing activity metrics error: duplicate key value violates unique constraint "cons_activities_unique"
    at Connection.parseE (/home/jaller94/Git/Matrix/matrix-appservice-slack/node_modules/pg/lib/connection.js:614:13)
    at Connection.parseMessage (/home/jaller94/Git/Matrix/matrix-appservice-slack/node_modules/pg/lib/connection.js:413:19)
    at Socket.<anonymous> (/home/jaller94/Git/Matrix/matrix-appservice-slack/node_modules/pg/lib/connection.js:129:22)
    at Socket.emit (events.js:315:20)
    at addChunk (_stream_readable.js:297:12)
    at readableAddChunk (_stream_readable.js:273:9)
    at Socket.Readable.push (_stream_readable.js:214:10)
    at TCP.onStreamRead (internal/stream_base_commons.js:186:23) {
  name: 'error',
  length: 307,
  severity: 'ERROR',
  code: '23505',
  detail: 'Key (user_id, room_id, date)=(@jaller94:localhost, INTEG-Q9cv0tMFJgpsU7pC38KZ1xyR6zBlK07W, 2020-03-16) already exists.',
  hint: undefined,
  position: undefined,
  internalPosition: undefined,
  internalQuery: undefined,
  where: undefined,
  schema: 'public',
  table: 'metrics_activities',
  column: undefined,
  dataType: undefined,
  constraint: 'cons_activities_unique',
  file: 'nbtinsert.c',
  line: '570',
  routine: '_bt_check_unique'
}

[Half-Shot]

I'm not too happy about silencing errors if we can fix them

[Half-Shot]

One more thing, would it be useful to have a test case for adding the same data twice into the datastore?

[jaller94]

Now with 100% more tests that actually already caught a mistake.