Consolidate button styles

This patchset will create consistent button styling according
to the styleguide and remove redundant button styles by creating
reusable button classes.

This patchset also implements a consistent strategy for including
icon assets. Currently, svg icons are imported as CSS background
images or inlined into the markup. Inlining an svg is preferred,
especially when used with buttons or links so the colors can
be easily changed for different states (hover, focus, disabled)
without having to request variants.
The icon provider allows us to inline svgs without cluttering
the markup. Webpack config was adjusted to use svg-inline-loader
when resolving svgs that are used by the icon provider directive.

- All svgs were optimized to remove unncessary information.
- Removed unused svg color variants
- Moved icons used by icon provider to separate directory to
  avoid Webpack parsing the files twice
- Small changes to navigation icons

Signed-off-by: Yoshie Muranaka <yoshiemuranaka@gmail.com>
Change-Id: I1ca214b74fc502e6b6e760cfee88b48110237c43
diff --git a/app/index.js b/app/index.js
index 1d54b45..e4061ed 100644
--- a/app/index.js
+++ b/app/index.js
@@ -55,6 +55,7 @@
 import serial_console from './common/directives/serial-console.js';
 import dir_paginate from './common/directives/dirPagination.js';
 import form_input_error from './common/directives/form-input-error.js';
+import icon_provider from './common/directives/icon-provider.js';
 
 import login_index from './login/index.js';
 import login_controller from './login/controllers/login-controller.js';