fix(avatar) revert back to defaulting to Gravatar

While the base URL remains configurable, this patch reverts back to using
Gravatar.

We noticed high latency with libravatar and contacted them. They are in the
process of migrarting to a better infrastructure (it's a single personal server
at the moment) so we'll re-evaluate once that has happened.

As for why not leave the default and change it on the meet.jit.si installation,
we don't want to kill their server :-)

react/features/base/participants/functions.js

     },
     (participant, store) => {
         if (participant && participant.email) {
-            return getGravatarURL(participant.email, store.getState()['features/base/config'].gravatarBaseURL);
+            // TODO: remove once libravatar has deployed their new scaled up infra. -saghul
+            const gravatarBaseURL
+                = store.getState()['features/base/config'].gravatarBaseURL ?? 'https://www.gravatar.com/avatar/';
+
+            return getGravatarURL(participant.email, gravatarBaseURL);
         }
 
         return null;