Implement app shell/template

[Salint]

The app shell should include:

• The sidebar.
• The content container shown on the right.

This ticket does NOT include:

• The home page

The side bar must be responsive, so that it's fully extended if a screen is large enough:

If the screen is smaller, it should be closed by default with the ability to extend:

And if the screen is extra small, the sidebar should be completely hidden but an extend button added everywhere, and when it opens it's an overlay.