Riot.js and Blaze are outdated

[loleg]

The current release is pinned to the latest version of Riot 3.x, which is now on version 6.x - see https://riot.js.org/
Our dependency on the no longer maintained riotgear package is blocking the upgrade. This is also forcing us to use an obsolete 1.8.5. version of the Blaze library (now @blaze/css).

The project's frontend components would need an upgrade to work with current versions of these libraries. The process should however be somewhat easier, now that a package management is in place.