WebUI Dark Mode: update .svg icons with embedded fill

This CL duplicates and updates the light mode .svg icons from
cr_icons_css.html to dark-specific equivalents. This was done
semi-mechanically with a bunch of sed hacking to copy and change the
fill color.  I also updated .md-select's icon as well.

I also renamed some icons that seemed to be overly presentation in their
naming (i.e. -white), changed the default --iron-icon-fill-color for
settings, deleted some unused shared variables, and added an error color
for settings.

Screenshots: https://imgur.com/a/MyI725P
Spec: https://docs.google.com/presentation/d/1kJoBzf_HGYK-_FAJPAjD67TW8224dCPLA6dAhlDmysk/edit?ts=5c485b44#slide=id.g364d7f9db1_0_0

R=aee@chromium.org
BUG=883049

Change-Id: I7709044a360a2e78136c2123dbe17ea0de662e37
Reviewed-on: https://chromium-review.googlesource.com/c/1431916
Reviewed-by: Esmael El-Moslimany <aee@chromium.org>
Commit-Queue: Dan Beam (slow until 1/29) <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625527}
28 files changed