Add display name change endpoint

[Nephyx]

Adds API endpoint for changing the display name of a user.
Related to Haider8/tmessage#39.

[vitokhangnguyen]

Hi @Nephyx,
I think that the route to change displayed name should be protected that...

1. Only a registered user with a valid token can successfully make a request to it
2. The registered user has to have their user_name matched with the user_name of the displayed name

If not, anyone can just use an Http Client (i.e.: Postman) to modify someone else's displayed name. Somewhere at the beginning of server.js, I believe there is this code can help this purpose:

// Add this middleware to routes need protecting
const basicTokenAuthentication = passport.authenticate("jwt", {session: false});

[Haider8]

Yes the route to change the display name should be protected