Add search support, improve styling

Add and implement a search UI to the web front-end of slicerappstore. (Search support within Slicer will be handled via a mixture of back-end features provided here and C++ code within Slicer; the 'empty' layout has not been changed.) Also, improve overall styling of the appstore.

(The latter started out as needed styling of the search field in order to include the icon which indicates its purpose, and got a little out of hand :-). The end result is that the appstore - due mainly to a bunch of new corner rounding - looks better in the context of the current midas styling, and the appstore widgets in particular have a new look that is significantly more aesthetic.)

Parts of the back-end PHP code are also cleaned up a bit.

This requires support in the slicerpackages extensions model; see midasplatform/slicerpackages#4.