Add horizontal overflow scroll to user content
Add horizontal overflow scroll to user content

It's possible for users to create content that expands
outside of the area where it is rendered. This is not
ideal. To solve this, an "overflow-x: auto" style is
added to all .user_content elements to supplement
the "max-width: 100%" style. If any content is wider
than the width of the container, it will add a scrollbar;
otherwise, it behaves the same as before, including
respecting text wrapping.

Test Plan:
On the syllabus page, using the RCE,
add really long content, like this table:

<table>
<tbody>
<tr>
<td><img
style="width: 1600px; max-width: 1600px;"
src="https://picsum.photos/1600/300" /></td>
<td><img
style="width: 1600px; max-width: 1600px;"
src="https://picsum.photos/1600/300" /></td>
</tr>
</tbody>
</table>

Save the page. It shouldn't overlap any of the
content on the aside, and should allow you to
scroll to see the overflowing content.

flag=none

Fixes LS-1243

Change-Id: I613513abe3fced52b432a41f249cb869cb29cf15
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/243733
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Jackson Howe <318c10a818c1b1f2ec50cdcb1827b2c6c5ebd7dc@instructure.com>
QA-Review: Robin Kuss <0b206b54367b2de0334896ea45826f0e3194bd35@instructure.com>
Product-Review: Alex Anderson <8690c85fe64134788579ce84d396a27368e019d5@instructure.com>

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