New book form: warn in advance if cover image is too big

[keul]

Describe the bug

When registering a new book, the upload allows to insert the cover image.
After spending time filling all required fields, I got back a web server 413 error (Request Entity Too Large)

Pressing "back" on my browser… I lost all of my data input.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://bookwyrm.it/create-book
2. Fill the form by providing a 1.3Mb image
3. submit

Expected behavior

Check in advance if the image is too large!

Instance

https://bookwyrm.it