config: default to 720p (#3269)

config.js

     // used by browsers that return true from lib-jitsi-meet's
     // util#browser#usesNewGumFlow. The constraints are independency from
     // this config's resolution value. Defaults to requesting an ideal aspect
-    // ratio of 16:9 with an ideal resolution of 1080p.
+    // ratio of 16:9 with an ideal resolution of 720.
     // constraints: {
     //     video: {
     //         aspectRatio: 16 / 9,
     //         height: {
-    //             ideal: 1080,
-    //             max: 1080,
+    //             ideal: 720,
+    //             max: 720,
     //             min: 240
     //         }
     //     }