Images: accept BMP format

[xaviermiller]

Is your new feature related to a problem? Please describe.
For now, RecipeSage does not support BMP file format. The images are silently ignored.

Describe the solution you'd like
Provide an error message or import / convert them

Describe alternatives you've considered
Now I need to convert BMP images to PNG before to load them in recipes

Additional context
Add any other context or screenshots/mockups for the new feature here.

[julianpoy]

Errors are only ignored during DB imports, but do throw an error when uploading individual images.

Support for BMP is a little challenging. See this issue for sharp (the conversion library we use): lovell/sharp#806

[xaviermiller]

OK, I will convert all my images to PNG.