"On-the-fly" stream rebroadcasting

Allow a stream to be received and override what is currently being played. The stream should be faded in and out according to the global preference.

How to enable it:
1) Auto-enable for all shows (global) - sets the default value for the "show" form
2) There is an option in the "show" form to enable/disable it

Security options to connect your stream:
1) global security: there is a master username/password that will get you into any show with this option enabled
2) Custom username/password: set username and password for this show
3) Connect using Airtime username and password

UI Changes:
*** Stream Settings ***
Section "On-the-fly live stream settings" with options:
 -- "Auto-enable for all shows"
 -- "Master username"
 -- "Master password"

*** "Add/Edit Show" form ***
 -- Checkbox for "Allow Live Stream Override"
 -- Security settings checkboxes:
     1) Connect using Airtime username & password (this follows the same security pattern for the shows: if no users are explicitly set for the show, then anyone with a valid airtime login can connect to the stream, otherwise if there are users assigned to the show, then only those users can connect)
     2) Custom: "Username" (text box) / "Password" (Text box)
 -- Static text field for: Connection URL

Example:
Airtime is playing a scheduled playlist, but user wants to interrupt it at some point with a live transmission from a club they are currently physically located at. The user has configured Airtime to allow a live transmission override on any currently playing show. The user sends a live stream to liquidsoap with the correct username and password, liquidsoap receives the stream on input.harbor(), fades out the currently playing stream using the global fade preference, and starts playing the live stream. When the live stream ends (user terminates it), the playlist resumes playing the track that should be playing at that time.

Update to the specs:
 - DJs using their own password or show-specific password should have their live stream faded out at the end of the show
 - DJs using the master user name and password should not have their stream cut off at the end of a show.
 - Anyone with the master password can override any other live stream that is currently playing.
 - An admin or program manager should be able to kick/disable the live stream for the show from the interface

These changes come from these comments:
"We've got a number of outside Guest DJs waiting to come on the radio station and we may have a slight problem. If one of those DJs forgets to switch off their live streaming feed, is there a way we can over ride it, as we've noticed at the moment we cannot over ride if their feed if they are still connected to the server online. Hence we can't send out recorded broadcasts uploaded in Airtime as when we click the "enable stream setting" it won't connect, as it says its in use.

Also, is there any way the enabled stream settings can be set to automatically switch over when a DJ has finished their show, and without giving them full permissions, ie. Managers, or admin, as we don't know them and obviously don't want them to have that much control over the Airtime."

Also, would you know if we can give them different encodements to log on to the streaming, so that we ourselves would have a master encode, which we could over ride theirs, if needed. We are using Sam Broadcaster, which works absolutely perfect, except for this small glitch."

"Now Playing" Frontend Widget needs to display something different when the live stream is happening.