Added a 404 page

This addition implements the design of the original 404 page.

change .BaseUrl to .BaseURL

Replaced .Url with .URL in sidebar.html

Add {{ .Hugo.Generator }}

Convert rem's to em's in breakpoints

Fetch Google fonts over https to avoid mixed contents warnings

Lowercase variables for .Site.Params

Variables under .Site.Params are accessed with all lower-case identifiers. Hugo won't find them with an upper-case first letter, regardless of how they're formatted in the site `config` file.

Replace `template` call with the `partial` call (new in Hugo v0.12)
for including partial templates

Conflicts:
	layouts/_default/list.html

CSS is now split into poole.css and hyde.css; chrome/sidebar.html updated.

Also cherry-pick:

  * poole/hyde@48b9d1f - Set a max-width for images