Non conforming espect ratios for level thumbnails cause trouble + author credit

[CardamomCake]

Now that custom levels can have custom thumbnails, this should meybe be looked at.

Might as well add a tag for the author, now that it is no longer a given that juki is the author

[74oshua]

Any reason not to crop thumbnails to a specific ratio on load? Or is there a reason to maintain flexible thumbnail sizes? If not, I can come up with a PR

[74oshua]

#269 fixes this