BitBucket import should use URL slug for project path
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### Proposal
The Bitbucket import page currently uses the repository's human-friendly name (e.g. "Example Repo") to pre-fill the project path field, but this field is used for the URL slug, and thus only supports url-friendly characters.
This makes the pre-filling useless in most cases - it would be more useful to pre-fill with the URL slug that Bitbucket uses (e.g. `example-repo`), if available.
Importing the project failed: This namespace has already been taken! Please choose another one.
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