Limit upload size of images

[joelvdavies]

Images should have a maximum upload size to avoid misuse, should be configurable like it is for attachments.

• Reject image creation requests that pass a specific size
• The maximum size should be configurable with the same default as the front end (currently 50MB at the time of writing)
• We may also wish to consider adjusting the max_size used in the underlying SpooledTemporaryFile used via the UploadFile object if possible (see https://stackoverflow.com/questions/73442335/how-to-upload-a-large-file-%E2%89%A53gb-to-fastapi-backend) as depending on Generate and store image thumbnails #30 we may need the image to be fully in memory anyway and this should make the upload faster
• Will need a larger and smaller image for e2e tests, one above the limit and one below (or we could potentially use faker/mock the data some other way?)