Improve App height

This PR tried to improve the iframe behavior of apps https://github.com/graasp/graasp-ui/pull/54

But this doesn't seem to work each time, probably because currently the app re-loads often.

And probably for this same reason, the first solution (which seems to be the easiest and working best) `window.frameElement&&(window.frameElement.style["min-height"]="600px")`. In the old Graasp, the height is successfully applied to the iframe. However in the new graasp, it doesn't. We should investigate to know whether it is due to the context, or React itself.

Could this be due to the loading mechanism we added? (hiding the iframe as long as it's not loaded)

The current and temporary solution is to set a fixed height for all apps.

Start by reviewing PR #54 and the iframe loading behavior described here, including when the iframe is hidden and shown. Reproduce the app reload case and compare it with the old Graasp behavior. Done means the app iframe receives a reliable height without relying on a fixed height for every app.

Written by the indexing model from the issue text.