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 }}

The original reference was to favicon.ico, when the provided image is .png

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