Fix #255: Crash when PersistentChat sends BYE instead of response

The log provided by the user shows that he is probably not allowed to
use PersistentChat. After a successful INVITE our first XCCOS command
causes the server to send a BYE, which causes SIPE to close the dialog.
Our 200 response to the BYE triggers a 481 response to which SIPE tries
to answer with a BYE without checking that the dialog is already closed.

Fix this by suppressing the BYE when dialog is NULL.
l10n: change translation URLs to transifex.com
srtp: parse encryption key from SDP
srtp: generate encryption key for each stream
srtp: offer encryption key when initiating a call
srtp: implement sipe_backend_media_set_encryption_keys()
notify: store ucPC2PCAVEncryption value form server
srtp: add sipe_backend_media_get_encryption_policy()
media: send encryption SDP attribute based on policy
media: add stream_decryption_keys list to sipe_media_private
media: refactor encryption compatibility check
srtp: negotiate encryption and apply keys to backend
srtp: encryption policies WIP