Use same format Unsplash uses for file name

[pierlon]

Summary

Update the filename format to the one Unsplash uses. File names are now in the slug format:

"User's name"-"image ID"-unsplash.jpg

This was initally flagged by the client.

Checklist

• [ ] My pull request is addressing an open issue (please create one otherwise).
• [ ] My code is tested and passes existing tests.
• [ ] My code follows the Contributing Guidelines (updates are often made to the guidelines, check it out periodically).