v0.13.8 - Canonical custom domains and asset filenames

Changes

• Custom domains: hostnames are trimmed, lowercased and stripped of a trailing dot before saving, and one hostname can belong to only one account platform-wide. domains_save on a hostname another account holds now returns a clear error instead of creating a second row. Punycode (xn--) hostnames are now accepted; a Unicode hostname is rejected with its xn-- form named in the message.
• Blueprint handles are unique per account at the database level. Two concurrent blueprints_save calls for the same new handle now converge on one record instead of one failing.
• Assets: a blank filename is derived from the uploaded file and made unique within its folder (logo (1).png). The 18 nameless assets in production were named from their files.

Fixes

• Uploading the same image name twice in an account no longer leaves a second, nameless asset behind.