webrtc, rtmp2: Fix parsing of userinfo in URI strings

While parsing the string, `gst_uri_from_string()` also unescapes the
userinfo. This is bad if your username contains a `:` character, since
we will then split the userinfo at the wrong location when parsing it.

To fix this, we can use the new `gst_uri_from_string_escaped()` API
that was added in
gstreamer/gstreamer!583

Fixes gstreamer/gst-plugins-bad#831

Part-of: <gstreamer/gst-plugins-bad!1481>