Add max-width to canvas layout container

fixes: CNVS-27360

This caps our nui Canvas layout to 1366px wide

Test Plan:
- Go to a page in canvas and stretch your browser window beyond 1366px;
you should notice that the content does not stretch the full width
- ACCEPTIONS: if there is a full-width app, these settings do not apply
- Go to Calendar or Conversations (these are full-width apps), you will
notice they still take up the full browser window
- Legacy is not affected by this change

Change-Id: I19c3257f4ff73e3c9432d78daf303384e558f8c0
Reviewed-on: https://gerrit.instructure.com/73155
Tested-by: Jenkins
Reviewed-by: Pam Hiett <phiett@instructure.com>
Product-Review: Pam Hiett <phiett@instructure.com>
QA-Review: Myller de Araujo <myller@instructure.com>

app/stylesheets/base/_layout.scss

@@ -24,6 +24,15 @@ $footer-links-border-color:         lighten($ic-color-dark, 16);
 }
 @else { body, html { height: 100%; } }
 
+@if $use_new_styles {
+  body:not(.full-width) .ic-app-container {
+    // We want to add a max size to most pages in canvas
+    // unless it's a full-width app
+    max-width: 1366px;
+  }
+}
+
+
 body {
   @if $use_new_styles == false { min-width: $min_main_width; }