Add "isSilent" flag to API which squashes notifications

[TheGiddyLimit]

This is required as when loading some importers (e.g. the Class Features importer), Plutonium eagerly pulls in side-data, and by extension, our addon data. This triggers a junk "could not import X because of bad requirements" message.

[Spappz]

So this a pre-emptive feature for data that hasn't yet been converted?

[TheGiddyLimit]

Nope, it's to allow Plutonium to access stuff in the background (which it already does) without throwing random messages at the user