Use crypto/rand to generate random suffixes in API keys. (#468)
Use crypto/rand to generate random suffixes in API keys. (#468)

Use crypto/rand to generate a random number between 0~9999 and use it as
suffix.

Log lines like this indicating our probing frequency is fast enough to
cause duplications:

```
error trying to create a new Admin API Key: pq: duplicate key value
violates unique constraint "realm_apikey_name"
```
google/exposure-notifications-verification-server,google/exposure-notifications-verification-server,google/exposure-notifications-verification-server,google/exposure-notifications-verification-server