Problem uploading when image directories are numbers

[saracarl]

Riksarkivet ran into a problem uploading a zip file of images. The upload hung, and they were sent no errors. When I looked at the upload processing, I found this:

Found 310 image files in /tmp/fromthepage_uploads/32003152/30002909/30002909 -- converting to a work
convert_to_work creating database record for /tmp/fromthepage_uploads/32003152/30002909/30002909
	convert_to_work owner = Riksarkivet Sverige
	convert_to_work collection = I demokratins namn – kvinnorna som krävde rösträtt
	convert_to_work default title = 30002909
	convert_to_work looking for metadata.yml in /tmp/fromthepage_uploads/32003152/30002909/metadata.yml
	convert_to_work no metadata.yml file; using default settings
	convert_to_work loaded metadata.yml values 

rake aborted!
ActiveRecord::RecordInvalid: Validation failed: URL must have at least one letter

This is the same as #3074, introduced by #2328

Discussed with @benwbrum and, since the core problem #2328 was trying to solve was slugs (aka URLs) being all numbers (and colliding with IDs), and we've run into problems with this twice now since we deployed that change, a validation is not the right place to put this. We should allow all numeric titles, but in the slug generator append something to make it a string rather than a number.

[saracarl]

@sylvieed take a look at this.