Will render if a file exists at: static/local/custom.css

Ref T563

I noticed most asset links use relative URLs, except for a few. This commit remedies this inconsistency.
OpenGraph embeds were not changed, because in my experience, some embed scrapers require absolute URLs.

Motivation: The site I use has an onion and a clearnet version, and currently, visiting the onion loads fonts, styles, and the favicon
over the clearnet.

This moves logic for determining whether or not to show a "Sign up" page
on /signup (i.e. because the `/` route shows another, non-signup page)
into the AppCfg.SignupPath() method. It also changes various signup
links to use this value.

This adds a Reader tab when necessary while logged in, and generally
keeps the navigation consistent for logged-in users, particularly in
regard to the Reader:

- Now includes user buttons and dropdown
- Makes header on user pages consistent with Reader page

This prevents the link from showing when an instance lands on the sign
up page anyway.

Ref T681

Ref T681 T680

Ref T681

This shows Drafts in the SimpleNav menu, when both enabled. It also
hides Drafts in the non-SimpleNav menu when disabled.

Ref T679

Ref T681

Also, refactor navigation bar template logic to be simpler and easier to
understand.

Ref T681

This provides easy navigation to the logged in user's posts, since
there's no direct link to their blog otherwise.

Ref T681

This adds a Sign Up link to site navigation and shows the
otherwise-landing page on /signup when in chorus mode.

Ref T681

This adds a new config value: `chorus` that signifies an instance is
more about the Reader view than individual blogs / writers. When
enabled, user navigation will show on all pages, including About,
Reader, and Privacy (ref T680).

It also uses different collection templates that keep the instance-wide
navigation at the top of the page, instead of the author's name --
again, branded more for the collective than the individual.

Ref T681

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.

Ref T576

This adds a "Reader" section of the site for admins who want to enable
it for their instance. That means visitors can go to /read and see who
has publicly shared their writing. They can also follow all public posts
via RSS by going to /read/feed/. Writers on an instance with this
`local_timeline` setting enabled can publish to the timeline by going
into their blog settings and choosing the "Public" visibility setting.

The `local_timeline` feature is disabled by default, as is the Public
setting on writer blogs. Enabling it adds a "Reader" navigation item and
enables the reader endpoints. This feature will also consume more
memory, as public posts are cached in memory for 10 minutes.

These changes include code ported over from Read.Write.as, and thus
include some experimental features like filtering public posts by tags
and authors. These features aren't well-tested or complete.

Closes T554

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