Add ability to disable h264 (#2061)

* doc: document disableH264 option

* [RN] Override disableH264 config option

We never want it disabled on mobile.

config.js

     // default and can be toggled in the p2p section.
     //preferH264: true,
 
+    // If set to true, disable H.264 video codec by stripping it out of the
+    // SDP.
+    //disableH264: false,
+
     // Desktop sharing
 
     // Enable / disable desktop sharing
         // is supported).
         preferH264: true
 
+        // If set to true, disable H.264 video codec by stripping it out of the
+        // SDP.
+        //disableH264: false,
+
         // How long we're going to wait, before going back to P2P after the 3rd
         // participant has left the conference (to filter out page reload).
         //backToP2PDelay: 5

react/features/base/config/reducer.js

     disableAudioLevels: true,
 
     p2p: {
+        disableH264: false,
         preferH264: true
     }
 };