Add incognito key to manifest?

[jaredhirsch]

Webextensions will soon (firefox 66) be disabled by default in private browsing windows, instead of enabled by default. To keep Screenshots working in private windows, we'll need to add the incognito key to the manifest. (I think it needs to be set to spanning to work in both modes?) More details here.

Not sure if this applies to system addons / privileged addons in the same way as third-party webextensions, but we should talk to mixedpuppy and land any necessary changes before the next export.

[ianb]

Fixed in #5294