Replace sidebar and add widget-type-specific homepages

[esimkowitz]

I think the next set of changes I want to make are to rework the sidebar and create widget-type-specific homepages. The new router layout mechanism added a bunch of hidden pre tags to the sidebar that are causing the accordion sections to display an extra lower border that I don't like. Rather than try and address that here, I am just going to rewrite this so it's more flexible. I want to be able to click on the widget type and show a homepage with just widgets of that type. Clicking on the chevron itself should be the only thing that collapses the section. This PR adds new mod.rs files for each widget type module so I can host this sub-home-page there and reuse the layout and logic from the current global home page.