Add 404 page which fits the rest of the theme

[ipetkov]

• Noting that (as of 0.14) zola does not pass much of a context when rendering the 404 page, so things like current URL/path aren't available
• To work around this the menu macro has been split into menu_for which accepts a parameter for which item is active.
• The previous macro continues to work as is, forwarding the results of figuring out which item is active to the new menu_for macro
• The added 404 page will then use the menu_for macro and pass in a blank item