Indicate the maximum file size for profile banners

At present the profile banner page under User Profile specifies the recommended size and supported formats however does not indicate the maximum file size.

Uploading an image exceeding the 10MB limit works however when attempting to save these changes the 'Unsaved Changes' will refuse to save the changes.

There is no indication in the UI that this is due to the file size, while I consider this a simply over sight Discord Support have said this is a feature request.

This is a REALLY simply fix:

- Modify the profile banner text to indicate the limit
- Probably a good idea to expand the text to indicate which animated formats are support (e.g Animated PNGs can be uploaded however will not be animated)
- Present the standard 'Your image too powerful' UI message when attempting to save a profile with a banner exceeding the limit or alternatively prevent it from uploading and showing in the preview