make gettext fails to extract sidebar titles conditionally

[sfc-gh-mpeters]

Issue
make gettext does not extract a sidebar title when the sidebar content includes any one of the following:

• List item
• Numbered item
• :raw: RST directive

To Reproduce
Steps to reproduce the behavior:

Add a sidebar similar to the following to an RST file:

  - <link>
  - <link>```

conf.py:

```locale_dirs = ['./locale']
language = 'fr'
gettext_compact = False```

Commands:

```make gettext```

**Expected behavior**
The sidebar title ("Related Topics") is extracted to the POT file along with the other strings.

**Environment info**
- OS: Mac Mojave
- Python version: 3.7.3
- Sphinx version: 2.2.11_2
- Sphinx-doc version: 2.0.1