Improve information about the level of support of extensions

[vmassol]

@tkrieck

So how about we change it to a filtering sidebar instead? This has the additional bonus of maintaining the overall design of the default search.

I think it looks better indeed. However, the sidebar is taking a lot of horizontal space so it would need to be collapsed by default, with a way to unfold it/fold it.

Thanks!

[tmortagne]

Loading... is now merged in master branch (so will be part of 16.8.0RC1).

There is no major change regarding the filtering in the EM main page (it’s just that an extension with at least one support plan attached is considered recommended). Created Loading... for that part.