profile: drop gravatar support

gravatar is owned by an external entity (Automattic) and we have an
unnecessary dep to this service. users can just upload a profile pic

src/constants.js

@@ -78,11 +78,6 @@ exports = module.exports = {
 
     AUTOUPDATE_PATTERN_NEVER: 'never',
 
-    // the db field is a blob so we make this explicit
-    AVATAR_NONE: Buffer.from('', 'utf8'),
-    AVATAR_GRAVATAR: Buffer.from('gravatar', 'utf8'),
-    AVATAR_CUSTOM: Buffer.from('custom', 'utf8'),   // this is not used here just for reference. The field will contain a byte buffer instead of the type string
-
     SECRET_PLACEHOLDER: String.fromCharCode(0x25CF).repeat(8), // also used in dashboard client.js
 
     CLOUDRON: CLOUDRON,