Resolve "Independence of deployment environment"

Closes #319

Everything is implemented and all tests are green, however this will be marked as a draft until I have thoroughly tested everything manually, since this is a very big merge request.

.env.unix.example

@@ -22,3 +22,5 @@ DATACITE_URL="https://api.datacite.org"
 DATACITE_PREFIX="10.1234"
 DATACITE_USERNAME="username"
 DATACITE_PASSWORD="password"
+USER_DB_USERNAME=root
+USER_DB_PASSWORD=dbrepo