Create add friends feature

[klam656]

Create the necessary functions, hooks and REST method and modifying the database model which help implementing the add friends features.

Modify database structure - I simply add to array in the user collection, which are friends, and friendRequests.

• friends (shows a list of friend username)
• friendRequests (shows a list of username that add you or would like to become friend with you)

Hooks:

1. useFriendRequests - (GET the all item/username from the friendRequests array)
2. useFriends - (GET the all item/username from the friends array)
3. useSearchUsers - (GET the all username that match the search prompt)

Functions:

4. addFriend - add username in the friendRequests array
5. acceptFriendRequest - add both users' username in their friends array
6. declineFriendRequest - remove username in the friendRequests

Contribute to #55