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