split expose_in_reviewer_tools to extensions and themes

[eviljeff]

Fixes mozilla/addons#16261

Description

Change expose_in_reviewer_tools from a False/True boolean, to a config to allow a policy to be selectively exposed for extension (actually non-theme) reviews, and theme reviews.

Context

As booleans are stored in mysql as tinyints, the database migration can directly alter the column to smallint without manually migrating the data - i.e. False will become 0, which is our constant for not exposed, and True will become 1, which is our constant for exposed in both extensions and themes.

Testing

• if you don't have any Cinder policies locally, sync your policies
• in django admin, change some of the policies to be exposed to both extensions and themes, some to extensions, and some to themes
• toggle off the waffle switch enable-policy-review-selection or you will hit [Bug]: themes are missing the review sections on review pages addons#16321
• in reviewer tools, navigate to the review page for an extension, and see only the policies for extensions and both are exposed for rejections and disable addon
• similarly, for a theme, see only the both and theme policies are available

Checklist

• Add #ISSUENUM at the top of your PR to an existing open issue in the mozilla/addons repository.
• Successfully verified the change locally.
• The change is covered by automated tests, or otherwise indicated why doing so is unnecessary/impossible.
• Add before and after screenshots (Only for changes that impact the UI).
• Add or update relevant docs reflecting the changes made.

[eviljeff]

This is because the scanners only operate over extensions right now