Improve security of components related to the external server

[BenjaminTanguay]

Points: 8

• [x] [Briar] Check if the usernames are already taken
• [x] [Briar] Use a non hardcoded password
• [x] [Briar] Fetch the information about other users through a route that is aware of if the other user is a contact or not
• [x] [Briar] Add hook on adding a contact to notify the Webserver
• [x] [Webserver] Hash the passwords for the database
• [x] [Webserver] Create an exist method to verify if usernames are already taken
• [x] [Webserver] Create the classes necessary to support user contacts

[Winterhart]

The four first issues are completed with my pull request #136