Challenge Participation Invitation in Development Mode

#### Current Behaviour:
Hitting the [invite_users_to_challenge](https://github.com/Cloud-CV/EvalAI/blob/master/apps/challenges/views.py#L1936) gives error of `Unable to find the sendgrid key` b'coz of [send_email](https://github.com/Cloud-CV/EvalAI/blob/master/apps/challenges/views.py#L2014) method as the key accessible in production only.

#### Feature Requested:
Just like the `email verification` and `password-reset` there should be similar method for sending invitation of participation by the host.

Start in apps/challenges/views.py at invite_users_to_challenge around line 1936 and send_email around line 2014. Compare the existing email verification and password-reset methods to understand the intended development-mode invitation flow. Done means a host can invite challenge participants in development without requiring the production-only SendGrid key.

Written by the indexing model from the issue text.