Add rate-limiting to OTP endpoints

[LoneRifle]

To prevent brute force attacks aimed at gaining unauthorized access, rate-limiting should be introduced to the endpoints that handle OTP requests. Given that we already use Sequelize to hold state, we should use Sequelize to track statistics relevant to rate-limiting.

[lamkeewei]

Thinking of introducing node-rate-limiter-flexible for this given that it has support for Postgres as a backing store. Thoughts?

[prestonlimlianjie]

+1 to the library. ~Given that we've decided to use SQLite for the repo's default storage, we can use node-rate-limiter-flexible's in-memory mode https://github.com/animir/node-rate-limiter-flexible/wiki/Memory~

@lamkeewei reminded me that Nest.js has a way of doing it within the framework