forward slashes in repository slug breaks workshop data submission

[tobyhodges]

See #11 for an example. Repo slugs submitted with a trailing / result in a misformed path for the resulting YAML data file. I think there are two ways to address this:

1. sanitize the input to strip the trailing slash. might be possible using the type: slugify option in the Staticman config, depending on how slug handles forward slashes.
2. place a prominent warning next to the form field that will encourage community members to avoid submitting slugs with slashes in them