Make TinyMCE cellpadding and cellspacing work

Fixes: CNVS-19489

Test plan:
- In TinyMCE, make 2 tables: one with cellpadding
  and cellspacing, and one without. Give both
  tables a border value, so you can see their
  dimensions.
- When you save your content, you should see that
  the cellpadding and cellspacing values are now
  being respected in published content.
- Note: this commit fixes everything except the use
  case where the user creates a table inside a parent
  table. Only tables that are direct children of
  the .user_content container were possible to
  target with this fix.

Change-Id: I28b589f38167549e1a4a046327f08fcaed08a3cc
Reviewed-on: https://gerrit.instructure.com/72715
Tested-by: Jenkins
Reviewed-by: Colleen Palmer <colleen@instructure.com>
Product-Review: Colleen Palmer <colleen@instructure.com>
QA-Review: Myller de Araujo <myller@instructure.com>

Author: Chris Hart

app/stylesheets/components/_g_instructure.scss

@@ -149,7 +149,7 @@
     margin-top: 1em;
     margin-bottom: 1em;
   }
-  table {
+  table:not([cellpadding]) {
     margin-left: 5px;
     td {
       padding: 2px;

app/stylesheets/components/_tables.scss

@@ -222,6 +222,23 @@ Enclose our standard Cog Dropdown within a **div class="ic-Table--actions** to r
 
 */
 
+// Base table styles for Canvas
+table {
+  max-width: 100%;
+  background-color: $tableBackground;
+}
+
+// Need to completely hide the border-collapse and
+// border-spacing defaults from TinyMCE so the
+// user-defined cellspacing attribute still works.
+:not(.user_content) > table,
+// However, for tables without cellspacing, we do
+// want to include the defaults.
+.user_content table:not([cellspacing]) {
+  border-collapse: collapse;
+  border-spacing: 0;
+}
+
 $sp-table: 14px; // Will replace this with global spacing variable, when it hits master
 
 //ic-Tab__Panel--is-open

app/stylesheets/vendor/bootstrap/_tables.scss

@@ -2,17 +2,6 @@
 // Tables
 // --------------------------------------------------
 
-
-// BASE TABLES
-// -----------------
-
-table {
-  max-width: 100%;
-  background-color: $tableBackground;
-  border-collapse: collapse;
-  border-spacing: 0;
-}
-
 // BASELINE STYLES
 // ---------------