Add navigation bar to each admin page

Previously, the navigation bar added in !28 (merged) only was on the index page, and restrained by its containing container element. This MR creates a new template for all admin pages and makes sure that each has the right navigation buttons. I've also now put the header in the template, so that we can make the styling of that uniform at some point, too.

While I was at it, I also made the reusable components.html components divs, not footers. A more logical element type, I believe. I also made the table styles consistent across the three pages. And I completed the navbar icons that were partially added in the original MR.

Signed-off-by: Georgios Andreadis info@gandreadis.com

Closes #27 (closed)