refactor guardian class for clarity & correctness

introduce NullUser to avoid type-checking
DRY up code
reduce number of multiple returns
remove some redundant/impossible logic branches
add pending test for possible bug
add test & fix for ability to flag archived posts
add #secure_category? method to topic class
Fix bug that prevented flagging of archived topics
Rename NullUser to AnonymousUser
DRY up can_<action>? methods
Fix some ownership logic, and a test, for Guardian
Merge pull request #879 from mattvanhorn/refactor_guardian

refactor guardian class for clarity & correctness