Previously, the URL of the dynamically-added pinned post on a
single-user instance would include the username, causing the link to go
to a non-existent page. This fixes that.

This shows About, Reader, Log out links on backend user pages when
logged in. It also adds "New post" buttons on the backend pages and
blogs.

Might as well do this right away, we're here anyway.

It now loads unconditionally and highlights all code blocks.

TODO: optimize to not load when there are no blocks

This is a first stab at having a configurable code highlighting option,
similar to the MathJax rendering option. This change makes a checkbox
in the settings for code highlighting using the highlightjs.org
library.
What works: code highlighting in multi-user env is like I would
expect. single and anon(?) needs work

Things to resolve/consider:
- does the .IsCode test for code highlighting need to stay? At least
this and that should use the same version of the highlight.js lib.
- can the common templating part be 'included' somehow?
- the anon vs single-user vs multi-user code is not completely
clear (to me)
- bring js to local instead of cloudfare cdn (perhaps combine with
MathJax)

This closes #15

- New editor nav
- New backend nav
- Support for drafts
- Different footers on backend