Move vendor Sass stylesheets into main Canvas compiler

Fixes: CNVS-16496

This change moves jQueryUI, Bootstrap and Flexbox grid
styles out of g_vendor.scss and into Canvas' main
compiled stylesheet, c-common.scss.

This will make it easier for us to gradually turn
the heavily customized Bootstrap and jQuery UI
modules into Canvas components.

(For example, we tried to move the Bootstrap buttons
stylesheet over into the /components directory,
which created dependency problems for the jQuery UI
buttons, which depend now on the Bootstrap styles.
With every stylesheet compiling into a single file,
this type of problem will be resolved.)

Test plan:
- This change should make no style changes whatsoever
  to Canvas. It just changes how the styles are being
  compiled.
- Open up Canvas and click around. Everything should
  be exactly the same as it was before.

Change-Id: I99c9ac86b5931d90f7b3802c73cd34a885127af9
Reviewed-on: https://gerrit.instructure.com/43372
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Colleen Palmer <colleen@instructure.com>
Product-Review: Colleen Palmer <colleen@instructure.com>
Reviewed-by: Ryan Shaw <ryan@instructure.com>
QA-Review: Chris Hart <chart@instructure.com>