Code review comment for lp://qastaging/~mhr3/unity-lens-files/fix-955229

There might be races with get_blacklisted_uris() being called before blacklist_proxy gets initialized, but like you said on IRC it's unlikely and the result will be an empty set being returned so I guess it's safe enough to ignore...

Some other comments for the sake of completeness:

 * You're assuming that if there's a URI it will be in the first subject template. This is true for Activity Log Manager, but generally it could be in any position (although indeed there should be only one them; two URIs in the same event template don't make sense).

 * You're ignoring single files being blacklisted. Is this because ALM only supports folders?

 * And, obviously, blacklists consist of much more than URIs, but yeah, probably not much point in hiding downloads by mimetype.