Unify Add plugin with Add theme: load tabs through ajax

[petercralen]

Make Add plugin page the same way as Add theme. So Upload plugin and tabs (features, popular ...) react without reloading whole page.

[j-falk]

Thanks for the ticket @petercralen, the upload and search part (I think) of installing a new plugin has been improved now for 4.6 with #35429:

See first attachment I uploaded above

but the tabs (features, popular ...) are still needing a reload on plugins while not needed for themes:

See second attachment I uploaded above

Not sure if there is already a ticket for the tabs, otherwise maybe this ticket could stay open to try address that?

[j-falk]

So couldn't find any other ticket similar to this so I updated the summary to be abit more specific what feature is still missing to unify Add plugin with Add theme.