Store nav items in database

[GAsplund]

Key features

• Closes As a maintainer, I want the links for navigation to be in a json/yaml, so that I can easily reuse them in different components without having to hardcode the data #62
• Store all nav items in the database
• Settings UI for configuring nav items (admin only)

Known issues

• There's really nothing stopping a nav category from having child items, even if the category (aka "parent") has a URL (which is considered an illegal state, but child items will just be ignored should that occur)
• There is no "dynamic" CSS sizing on the navbar width, meaning that the content could look cramped before switching to hamburger menu

[GAsplund]

Okay, I'm not crazy. It seems like editing the navbar to be dynamic messes a lot with static routes. I think it propagates out to every other page but mainly is because (I think) that the root error page needs to be 100% static. I will look into this later when I have the time.