add url options to open an initial channel

[binsky08]

Open the quassel-webserver website with a preselected query or channel like:

• query example: https://quassel.example.com/#channel=felix&network=hackint
• channel example: https://quassel.example.com/#channel=#exim&network=Freenode

[scj643]

With this we can actually also add a back and forward button handling by updating the hash on channel change and check it on hash change.