Make it simpler to subclass BasicTemplate

[philippjfr]

The idea of a BasicTemplate with its rendering areas like main, sidebar, header and dialog has made it very easy for users to build nicer looking apps. However in many cases a user may want to define their own template or even extend an existing template to have greater control of the look and feel of their application. Right now there are quite a few difficulties doing this which you have to hack around to even make this possible. I'll list some of these here:

1. A subclassed template will still look in Panel's dist directory (or CDN) for the CSS and JS files declared on the template. We may have to provide a way to easily bundle these files or simply falls back to inlining them if they could/can not be bundled.
2. The jinja2 templates are not written with extensions in mind, we should make better use of block macros to allow users to overridde just certain parts of a template without overriding the whole thing.
3. There is currently no documentation for extending a template.

I'm sure there are many more difficulties which we should address.

[MarcSkovMadsen]

My thought has been that the components of the template should be refactored into standalone components using ReactiveHTML.

I believe it's now possible and very easy to create Header, Sidebar, Footer, Maximize, ReactGrid, Golden etc. components. That would also enable them to be dynamic (I.e. you can change the sidebar components, the header components and title) etc.

We should still provide templates. But those templates are build from the above components.