Add missing icons in navbar for installed modules

Targeting installed modules separately from core modules so we can pull their asset icon and use it as an inline icon in the navbar.
Adding reusable greyscale class for module icons

This class can be used to style custom module icons to match zpanel core theme style icons.
Adding missing custom module icon support

Adding missing custom module icon support in sidebar to accompany navbar support. Also added transparent class to match current core module icons. Also misc code formatting.
Adding transparent class for sidebar icons

Transparent class for custom sidebar icon support to match current core module icon styles.
Fixing module grid layout

Fixing issue when module grid wraps icons it breaks the layout. Took the float up a level, set the contents inline, and vertically aligned the inline elements with a bit of padding below for readability. Also greyscaled the icons to match the core theme.
Merge pull request #155 from stokes84/master

Improved UI by adding module icons to missing menus as well as enabling 'greyscale' transformation for the default theme to be inline with the current theme icons.