Don't wait for `load` event to render chatrix onto the page

[psrpinto]

Since #160, when rendering the block on the site front, we're waiting for the load event instead of DOMContentLoaded. This means that we're introducing an extra delay which is not ideal. More details here.

We should look into how to properly fix the issue for which #160 is a workaround, and revert back to DOMContentLoaded.