API: logout route

[HarshDobariya79]

This is a sub issue of #9

Tasks

• There should be a logout route where the header will contain a refresh token.
  • E.g. Authorization -> Bearer TOKEN_VALUE
• The refresh token must be invalidated by adding that token to the blacklist collection.

The blacklist collection schema:

Field	Description
timestamp	Expiry timestamp of refresh token. (It is necessary to store the timestamp with the refresh token to make it easier to clear up already expired refresh token in regular intervals.)
token	The actual refresh token