fix(modules/unifi): proper monitoring and latest version

They've recently removed from nixpkgs the version of mongodb that was
used by unifi. I updated to the latest version (7) and did the migration
of the DB manually (see https://github.com/NixOS/nixpkgs/pull/207382):

```
nix-shell -p mongodb-3_4 mongodb-tools
mongod --dbpath /var/lib/unifi/data/db  --logpath /var/log/unifi/repair.log --repair
mongod --dbpath /var/lib/unifi/data/db  --logpath /var/log/unifi/repair.log --journal --fork
mongodump --out=/root/mongodump
pkill mongod
exit
nix-shell -p mongodb-4_2 mongodb-tools
mv /var/lib/unifi/data/db /var/lib/unifi/data/db_bak
mkdir /var/lib/unifi/data/db
mongod --dbpath /var/lib/unifi/data/db  --logpath /var/log/unifi/repair.log --journal --fork
mongorestore /root/mongodump
pkill mongod
```

Once this was done, the exporter was also broken, has it has been
renamed. There are two different services for it in nixpkgs:
`services.unpoller` and `services.prometheus.exporters.unpoller`. Only
the last one works.

From what I can tell, everything is working now.

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