Rethink navigation/UI, including mobile

[jacobobryant]

After #39 is closed, add a link below Posts for each site you've created:

[Posts]
├── [My Site]
└── [Another Site]
[Sites]
[Newsletters]

The link could look something like /app?site-id=..., i.e. we just pass in a query param to the posts page and use it to filter the posts shown. (that also reminds me we should fix that /app so it's just /posts or something... should make a separate issue for that)

[jacobobryant]

Perhaps we should wait until #43 is closed, since we'll want to update the navigation once that's finished anyway.

[jacobobryant]

Now that platypub supports custom item types, I think the data model won't change too much more. It would be worth thinking about how to structure Platypub's UI. I chose the current navbar structure (shown below) out of expedience, not necessarily because I thought it was ideal:

[Posts]
├── [My Site]
└── [Another Site]
[Sites]
[Newsletters]

And the mobile navigation is particularly bad since it doesn't have the navbar and I just wanted to get something done with as little effort as possible.