* Fingerprint DBConnection.ConnectionError in Sentry

* Check events before creating a site

* Enable sites limit screen

* Remove debugging remnant

* Fix buggy assertions

This wasn't doing what expected:

  iex(1)> Repo.exists?(Plausible.Site, domain: "foo")
  [debug] QUERY OK source="sites" db=0.6ms idle=1906.2ms
  SELECT TRUE FROM "sites" AS s0 LIMIT 1 []

* Encapsulate check to satisfy credo

* Use less technically involved error message

* Bring back e-mail to the limit error message