Setting `MONKEY_MONGO_URL` env variable to non-default value results in island exiting with error

## Describe the bug
The mongo URL is configurable via the `MONKEY_MONGO_URL` environment variable. However, Flask-Security also has a connection to mongodb, and it uses hard-coded values instead of the connection details from `MONKEY_MONGO_URL`. Thus, if `MONKEY_MONGO_URL` differs from the defaults, the server will error out because one of the connections to mongo will fail.

Introduced in 564c15dc1cc2395151a545ea89f1d9ff9260becc #2157

## To Reproduce
Steps to reproduce the behavior:
1. Setup and run mongodb on a different port (e.g. `27019`)
2. Set MONKEY_MONGO_URL environment variable with the port (e.g. `mongodb://localhost:27019/monkey_island`)
3. Run the island
4. Observe that the island exits because it failed to connect to mongodb

## Expected behavior
I expect both connections to mongodb to use the same connection details.

## Machine version (please complete the following information):
- OS: Windows or Linux