use url safe jingle session ids

Movim does not like slashes (/) in jingle session ids.
When proposing a session called 'wBKabx1kRIfkgNxAShip/w' Movim will
accept (proceed) a session called 'wBKabx1kRIfkgNxAShip' which the initiator of course does not know about. (Conversations will get stuck at ringing/discovering devices)

This is likely because a click on 'Reply' (accept call) in Movim opens upa pop up window where both the full jid as well as the session id are transmitted as part of the URL.

(Full jids can contain more than on slash btw)