Project descriptions mishandle some table tags and attributes

[jacius]

Describe the bug

If a project description includes certain HTML tags and attributes in a <table>, those tags and attributes will be stripped or incorrectly displayed on the project page. Specifically:

• <caption>. On the project page, it will be converted to literal text (<caption>...</caption> in the page source) instead of an actual <caption> element.
• <th scope="...">. On the project page, the <th> tags will have no scope attributes in the page source.

This issue is particularly significant because this markup is important for accessibility.

To Reproduce

1. Create or edit a project.
2. In the Description field, switch the editor to HTML mode, and enter code for a table, such as:

<table>
  <caption>
    Example table caption
  </caption>
  <thead>
    <tr>
      <th scope="col">Name</th>
      <th scope="col">Round 1</th>
      <th scope="col">Round 2</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <th scope="row">Alice</th>
      <td>1</td>
      <td>2</td>
    </tr>
    <tr>
      <th scope="row">Bob</th>
      <td>3</td>
      <td>4</td>
    </tr>
  </tbody>
  <tfoot>
    <tr>
      <th scope="row" colspan="2">Total</th>
      <td>10</td>
    </tr>
  </tfoot>
</table>

3. Save the project and view the project page.

Expected behavior

The project page should include a table with a proper <caption> element, and all <th> tags should have their scope attributes when the page's HTML source code is inspected.