R1:88d9366701c5

Allow users to search for signatures across documents

Summary:
Ref T3116. You can already search for sigatures on a specific document, but allow them to be searched across documents too.

In particular, this lets users answer questions like "Which of these 5 documents has alincoln signed?" / "Has alincoln signed all the stuff I care about?" / "who has signed either L5 or equivalent document L22?", etc.

Test Plan: {F171658}

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

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