Jetpack: Cancellation survey should encode URL properly

[beaulebens]

Steps to reproduce

1. Buy a plan for a Jetpack site which is installed in a directory (not at the root of a domain)
2. Go through the process of canceling the plan
3. Observe incorrectly-encoded URL (e.g. we're displaying the slug version, which contains ::, not a correctly-encoded, real URL, which would change that to /).

Context / Source

#manual-testing