Consolidate Macro loading

Summary:
Fixes T2820

Grepped for PhabricatorFileImageMacro, a common approach to load image macros from storage. Cleaned up file loading too (in most cases where I could be sure that I won't break anything).

Did not touch the add_macro.php util script, since many users will assume the user ubuntu or ec2-user to run the script. Add no sessions and no CSRF protection measures...

Test Plan:
Browsed around all kinds of places. Created and looked at memes, created and edited macros. Used them in Remarkup (with flushed cache). Used macro.query, verified it did not crash (that's always a good sign).

Could not verify object handles, since I have no idea where they appear right now.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2820

Differential Revision: https://secure.phabricator.com/D5418