(8.5.4) GH-2166: Update name of Unknown category

[benstrumeyer]

• Have you followed the guidelines in CONTRIBUTING.md?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you added an explanation of what your changes do?
• Does your submission pass tests?
• Did you lint your code prior to submission?

• Update name of unknown category

Ticket: https://ghostery.atlassian.net/browse/GH-2166

[wlycdgr]

Same deal here as with GH-2178: let's update the variable names in the code to try to keep everything consistent. There are a lot of places where we use unknown but the large majority of them look like they should be a straightforward search&replace situation.

Bigger picture, we should think about whether there might be a safer and more efficient way we can keep this stuff in sync. It would def be better if we could make these kind of copy updates without needing to manually update a bunch of code like this.

[wlycdgr]

The panel's unkown.svg file also needs to be renamed - right now the icon is broken cos it's looking for unidentified.svg

[wlycdgr]

LGTM