Cannot navigate to a general linter message with the keyboard

[kumar303]

Describe the problem and steps to reproduce it:

• After general linter messages can be displayed (see https://github.com/mozilla/addons-code-manager/issues/805), browse an add-on that has a general message, i.e. one applying to the entire add-on. For example: 'Validation generated too many errors/warnings so 163 messages were truncated'
• Press z to navigate to the first linter message on the page

What happened?

The general message is skipped

What did you expect to happen?

The general message should be the first one selected

Anything else we should know?

When wrapping around to the end of all messages and starting over, the general message should be selected again.

┆Issue is synchronized with this Jira Task

[kumar303]

Skipping linter messages could be a security risk so we should fix this.