Added purge to find/update methods. Removed purge buttons.

This patch removes the 'Purge Cache' buttons from the Extension Manager's 'Update', 'Discover' and 'Install languages' views.
The purge functionality is now added to the find/discover methods.

Why?
Now everybody needs to purge first and then do the find/discover.
There is no situation where you use one or the other alone. So it is pretty useless to have them as separate buttons.
So now, if you click on the find/discover button, the cache is purged first, and then the find/discover takes place.