Fixes #3492 joining after document is renamed inside editor

Bug:
1. Open document
2. Rename in Collabora Online using input in the title bar
3. In the new tab open the same document from file picker in NC
Result: 2 sessions are in different instances of the same document

getCallbackBaseUrl() puts "/" at the end, but WOPISrc was adding
additional one:
const wopiurl = getCallbackBaseUrl() + '/ ...

Then reconnected session after rename had single "/index.php"
but new session used "//index.php" what caused that COOL server
didn't consider both as the same identifier

Signed-off-by: Szymon Kłos <szymon.klos@collabora.com>

src/helpers/url.js

@@ -34,7 +34,7 @@ const getSearchParam = (name) => {
 
 const getCallbackBaseUrl = () => {
 	const callbackUrl = Config.get('wopi_callback_url')
-	return callbackUrl || window.location.protocol + '//' + window.location.host + getRootUrl() + '/'
+	return callbackUrl || window.location.protocol + '//' + window.location.host + getRootUrl()
 }
 
 const getWopiSrc = (fileId) => {