Security: Force TURN setting was inverted

In making the settings more positive, we flipped 'disable peer to
peer' but didn't change the caption, so the setting was inverted
from what it was labelled as.

Also, forcing p2p isn't the inverse of forcing turn.

Change the setting to be 'allow p2p' and the label to match.

src/MatrixClientPeg.js

@@ -177,7 +177,7 @@ class MatrixClientPeg {
             userId: creds.userId,
             deviceId: creds.deviceId,
             timelineSupport: true,
-            forceTURN: !SettingsStore.getValue('webRtcForcePeerToPeer', false),
+            forceTURN: !SettingsStore.getValue('webRtcAllowPeerToPeer', false),
             verificationMethods: [verificationMethods.SAS]
         };