Fix external links for sites with no pages

[pdmosses]

Fix #1020

1. Move the display of nav external links from _includes/nav.html to _layouts/default.html.
2. Replace unless include.key by if site.nav_external_links.
3. Wrap the body of if site.nav_external_links in <ul class="nav-list">…</ul>.

To test this PR:

1. Add to _config.yml:

   defaults:
     - 
       scope: {path: ""} 
       values: {nav_exclude: true}

2. Rebuild the site.
3. Check that the nav panel displays only external links.

[pdmosses]

I've added a regression test for this issue.

[mattxwang]

Thanks for taking a crack at this @pdmosses. I have a bit of limited time right now (I'll try to review / merge the other PRs later today/tomorrow), but just to check my understanding: the problem in #1020 is that pages_top_size in default.html is 0 when there's only one index page, so the nav isn't even being rendered?

I wonder if it's bad (for any reason) for us to have two different ul items for the nav. I'll have to think about it for a bit.

[pdmosses]

... just to check my understanding: the problem in #1020 is that pages_top_size in default.html is 0 when there's only one index page, so the nav isn't even being rendered?

It's 0 when all ordinary pages are excluded from the nav panel by nav_exclude: true. The index page is not automatically excluded (unless it has no title).

Anyway, moving the external links code to the default layout should increase the modular flexibility of the latter.

I wonder if it's bad (for any reason) for us to have two different ul items for the nav. I'll have to think about it for a bit.

I didn't notice any difference in the appearance. But if we wanted, I think it would be easy enough to strip the uls there, and enclose the resulting sequence of lis in a single ul.

[mattxwang]

Gotcha. Have done a more thorough look through, and I think I better understand this change; in particular, I feel like the previous approach "overloaded" the include key (using its existence to "check" if we were in the non-collection nav, when ideally, external links are considered separate from page nav elements).

Later down the line (perhaps motivated by #959), I think we should eventually move this to its own sandboxed file/include. Haven't gotten around to implementing it yet though, so let's not put the cart before the horse. LGTM!

(and, thanks for pointing the original user to your test branch; seems like it's resolved the original issue, which is great!)