Allow WebSocket connections on more URL's

This patch adds the concept of ast_websocket_server to
res_http_websocket, allowing WebSocket connections on URL's more more
than /ws.

The existing funcitons for managing the WebSocket subprotocols on /ws
still work, so this patch should be completely backward compatible.

(closes issue ASTERISK-21279)
Review: https://reviewboard.asterisk.org/r/2453/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@386020 65c4cc65-6c06-0410-ace0-fbb531ad65f3