Update Riot to 1.5.15
Changes:
- Security:
- The jitsi.html widget wrapper introduced in Riot 1.5.14 could be used to extract user data by tricking the user into adding a custom widget or opening a link in the browser used to run Riot.
- Bugfixes:
- Upgrade React SDK to 2.3.1 for Jitsi fixes
- Fix popout support for jitsi widgets