Image Macro Generator

Add a feature to generate image macros based on image files that have been uploaded.

This should probably be stored in an auxiliary table. It could have a foreign key to a File and then a caption/name + s3_id

Then we could give a unique URL for each image macro, and have an link on a file page (if macros exist) to view the macro list.

The issue names no files or tests, so first locate the existing File model, upload handling, and file page routes. Read how files are stored and exposed, then define the auxiliary image-macro records and their unique URLs. Done means uploaded images can have macros, each macro is viewable by URL, and file pages link to the macro list when one exists.

Written by the indexing model from the issue text.