Fix max filesize handling
- mention that per-file limit is only clientside (besides saving each
file to a temporary directory it depends on the environment if we can
get the size so we simply avoid it)
- set flask's MAX_CONTENT_LENGTH to the total filesize limit. this is
actual request body limit but since it's usually in tens/hundreds of
megabytes
- set the dropzone max size to the per-file or global limit, depending
on what's smaller. if no limit is set use 10GB since we need to set
*some* limit