Add schema validation

[anandbaburajan]

We need to have proper schema validation for cases like making sure the Poll choices are UNIX timestamps, the intervals are neither too small nor large, etc. Going with joi might be a good idea.

[anandbaburajan]

I looked more into this and here's what I learnt:

1. Using joi for validation at the 'request' level won't be necessary since mongoose' built-in validators take care of the types and most of the validation would be done on the client - which is the primary consumer of our API. For example: for choices, we could check if a choice is greater than the current Unix timestamp but that won't be necessary since the client would take care of that anyway.
2. Regarding intervals, the backend shouldn't have any constraints on the min & max intervals. There could be users who might want to have both 10 minute meets or 1 day hackathons. For the MVP, the client could let users choose a interval in a certain range like discussed here.