Multiple users support

- Keeping a map of username to connection pointer
- Having a register button to add username
- To send to a user, get the connection from the map and send it to that
connection's channel
- The HTML page has a target user text box to denote what user the message will
go to

Reviewed by:
Testing:
Tickets:
Backport:
Added error checking for condition when target user is not present in the map

Reviewed by:
Testing:
Tickets:
Backport: