Added image filesize validation
Added max_filesize to all image uploader views
Fixed display of hidden fields on image add view

Same as #668 but for the old image add view
tweak to validator message. passing through max size vars
Refactored image validation into a custom field

This fixes issues with Django closing the image file before running
validators.
This may give a slight improvement in upload speed as the file doesn't
keep having to be reopened.
This unifies our image validation with Django's image validation so
error messages could be shared (not implemented yet though).
The code is more maintainable (the validators no longer need to be
called manually from the multiple image uploader for example)
Check image file size before the format

Checking the file format may take a long time if the uploaded file is
very big so it makes sense to check the file size first to prevent the
server being held up unneccessarily.
started adding pre-upload validation to images
started adding pre-upload validation to images
updates to how error message and accepted file type/size are mentioned by jquery upload plugin
less poluting method of variable scoping
WagtailImageField help text. Some refactoring
Added separate error when image format is known
unifying of messages and added to uploader
Clarity/translatablity improvements. Fixed test
Fixed allowed extensions in multiple image uploader
Fixed reference to ALLOWED_EXTENSIONS_TEXT
Fixed JavaScript error when max_filesize is None
Cleaned up allowed extensions regex
Fixed crash when no file selected
Exclude focal point from multiple image uploader