Fix display of site URL on disconnect

[jaswrks]

Fixes #8912

Changes proposed:

In the Jetpack Disconnect Dialog:

• Use /::/g to replace all occurrences with /

Testing instructions:

• Clear your browser cache.
• Checkout this branch and run npm run build.
• Connect a site that's at least two subdirectories deep; e.g., example.com/wp/personal
• Then, from the Jetpack Dashboard, click 'Manage site connection' to disconnect it.
• Observe; instead of :: you should see the correct / separators in the URL.

WIth the fix in this PR, all instances of :: are replaced with /

Before it was only replacing the first occurence of ::

Proposed changelog entry:

• Dashboard: Fix :: separators (should be /) when disconnecting a site in a subdirectory.

[designsimply]

I tested on a site with WordPress installed two subdirectories deep running WordPress 4.9.4 and Jetpack c777b1f (fix/disconnect-site-url branch) by going to WP Admin > Jetpack > Connections and clicking "Manage site connection" and in my case the :: separator is still there for the 2nd occurrence:

^{Seen at http://madefortesting.com/temp/subdir/wp-admin/admin.php?page=jetpack#/dashboard using Firefox 58.0.2 on macOS 10.13.3.}

[jaswrks]

@designsimply Thanks for the review, I appreciate it :)

I'm sorry, I'm adding two additional steps to test this:

• Clear your browser cache.
• Checkout this branch and run npm run build before testing.

[designsimply]

Re-tested as discussed on Slack with the jetpack.zip provided (thank you 😊) and confirmed the separator for the 2nd subdirectory is now a / instead of ::.

^{Seen at http://madefortesting.com/temp/subdir/wp-admin/admin.php?page=jetpack#/dashboard using Firefox 58.0.2 on macOS 10.13.3.}