Secret Document Objectives

Adds new steal objective "Secret Documents"
-A set of secret documents spawn in the vault's filing cabinet, but even syndicate documents qualify as valid steals

Adds the exchange objective for traitors
-If there are at least 6 traitors, two of them will be selected to make an exchange of secret documents
-There is a low (20%) chance that one of them will have an additional backstab objective, where they will require both sets of documents to win
-Sending Centcom syndicate documents now gives the station a large supply point reward
-Removed some duplicate code
-Made the syndicate document names appear like codenames
Merge pull request #519 from Ikarrus/documents

[MAP] Secret Document Objectives