Search for tests without an associated web feature

[jcscottiii]

The introduction of #3663, #3664, #3665, and #3667 enabled searching for tests by web-feature. It would be great to have some way to search for which tests that do not have a web feature associated with it.

On the backend, a new method to check if a given test path exists within WebFeaturesData can help with this.

On the frontend, the user should be able to type an atom such as:

• no:feature
• !feature, or
• -feature