Reduce pad's hash size

Etherpad has a limitation of 50 characters for it's pad ids. Although
our SHA1 hash function generates 40 characters length values, after
prefixing (Meteor's instance id) and suffixing (closed captions support)
the pad id we can reach an invalid size.

bigbluebutton-html5/imports/api/common/server/etherpad.js

@@ -6,6 +6,7 @@ import createCaptions from '/imports/api/captions/server/methods/createCaptions'
 
 const ETHERPAD = Meteor.settings.private.etherpad;
 const BASE_URL = `http://${ETHERPAD.host}:${ETHERPAD.port}/api/${ETHERPAD.version}`;
+const HASH_SIZE = 36;
 
 const createPadURL = padId => `${BASE_URL}/createPad?apikey=${ETHERPAD.apikey}&padID=${padId}`;
 
@@ -15,7 +16,7 @@ const appendTextURL = (padId, text) => `${BASE_URL}/appendText?apikey=${ETHERPAD
 
 const checkTokenURL = () => `${BASE_URL}/checkToken?apikey=${ETHERPAD.apikey}`;
 
-const hashSHA1 = (str) => sha1(str).toString();
+const hashSHA1 = (str) => sha1(str).toString().substring(0, HASH_SIZE);
 
 const checkServer = () => {
   return new Promise((resolve, reject) => {