Standardize file fields in Media models

[bruvellu]

The names of the file-related fields are confusing: file, sitepath, coverpath, filepath (old). Their upload_to directories are also different. This has led to some difficult to catch behaviors. Files being uploaded or saved to different directories, or the metadata update being applied to different files (leaving others untouched).

The first step to clear this out is to define clearly the model fields. I'm torn between more descriptive names (but longer), or simpler short names:

Current	Proposed	Name	Image	Video	Comment
file	file	original	any/any	any/any	Any format or width within the file size limits.
-	file_large	large	JPG/2000px/q90	MP4/1920px(1080p)/2000k	Default formats at a large size
sitepath	file_medium	medium	JPG/1000px/q70	MP4/1280px(720p)/1000k	Default formats at a medium size
-	file_small	small	JPG/500px/q70	MP4/640px(480p)/600k	Default formats at a small size
coverpath	file_cover	cover	JPG/750px/q70	-	Snapshot for poster images
filepath	oldpath	-	-	-	Old file paths, needed for later

See list of common resolutions.

[bruvellu]

Done 1b2726a9c5b6a7ec391bacef8d0e71038268c642