Run DB Install check in WP Admin and WP CLI

[tomjn]

Related to #1284

Check the tables exist to avoid problems when the normal activation mechanism isn't used, e.g. MU plugins.

This PR checks on WP Admin and WP CLI so as to avoid constantly checking on busy frontends or endpoints

Checklist

• Project documentation has been updated to reflect the changes in this pull request, if applicable.
• I have tested the changes in the local development environment (see contributing.md).
• I have added phpunit tests.

Release Changelog

• Fix: Describe a bug fix included in this release.
• New: Describe a new feature in this release.

Release Checklist

• This pull request is to the master branch.
• Release version follows semantic versioning. Does it include breaking changes?
• Update changelog in readme.txt.
• Bump version in stream.php.
• Bump Stable tag in readme.txt.
• Bump version in classes/class-plugin.php.
• Draft a release on GitHub.

Change [ ] to [x] to mark the items as done.

[kasparsd]

Looks good!