mod_admin_modules: Cleanup code

Rename all functions, and html elements to confrom to Zotonic's naming convention.
Other changes include:

* Change table row background colour when clicked. This is to indicate that is has been selected
* When an already highlighted row is clicked, remove module from list of modules to install
* Dsiplay a helpful message while the module list are fetched from the server
* Do not display growl message after fetching the latest modules from the server
* The ZMM table is no longer striped, as it was interfering with the highlighting of tables
* Add "install" button when a row is clicked/selected. This is only done when the "install" button is not visible