[C2] Add commercial plugins differenciation (page/tag)

[ywarnier]

We want Chamilo to be more accessible to commercial providers who would like to offer their plugin for a cost, but we don't want these to mix with the default, free, plugins that come with the Chamilo package, all inclusive.

So the idea would be to have a separate tab in the plugins page, that will allow people to check out more plugins that may add a lot of value but come with a cost.

However, most commercial plugins are free but link to optional commercial services, like Wiris, OnlyOffice, Zoom, etc. These need to be distinguished properly.

I believe the way we identified third party plugins in Chamilo 1, with a small diagonal ribbon on top stating "Third party" or "Official", is OK to let people identify correctly which is what, but it identifies something else (if it was uploaded later on or if it was there initially) maybe a second ribbon is necessary like "100% free" or "Freemium".

Also, each plugin should have its documentation in a format that allows for some promotion. The README.md format we used in 1.11 seems fine (it's a bit tricky for images but with a bit of documentation, that should work). This requires to re-enable the README viewer.