feat(ui-elements,ui-themes): add link decoration brand variable to canvas theme

This variable should be set to 'underline' in canvas if the
"underline all links" feature is enabled.

TEST PLAN:

Verify that there is no change to the Link component examples

Change-Id: I14ad18c33007c0e1b2f049a9f18debb665f8dcd6
Reviewed-on: https://gerrit.instructure.com/158531
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Product-Review: Jennifer Stern <jstern@instructure.com>
Tested-by: Jenkins
QA-Review: Dan Sasaki <dsasaki@instructure.com>

Author: junyper

packages/ui-elements/src/components/Link/theme.js

@@ -58,6 +58,7 @@ generator['canvas-a11y'] = generator['canvas-high-contrast'] = function ({ color
 generator['canvas'] = function (variables) {
   return {
     color: variables['ic-link-color'],
+    textDecoration: variables['ic-link-decoration'],
     hoverColor: darken(variables['ic-link-color'], 10)
   }
 }

packages/ui-themes/src/canvas/base/index.js

@@ -58,6 +58,7 @@ export const brandVariables = {
   'ic-brand-font-color-dark': colors.textDarkest,
 
   'ic-link-color': colors.textBrand,
+  'ic-link-decoration': 'none',
 
   'ic-brand-button--primary-bgd': colors.backgroundBrand,
   'ic-brand-button--primary-text': colors.textLightest,