Update/newsletter intergration subdirectory support

[reintjanhoiting1]

See #16808.
Fixes #17027.

Changes proposed in this Pull Request:

support subdirectory for wordpress

Does this pull request change what data or activity we track or use?

No

Testing instructions:

test case 1

• Go to Posts > Add New
• Search for ‘Newsletter Sign-up’.
• Select the form block in the Gutenberg editor.
• collapse the "Newsletter Integration" on the right side
  
• Click install plugin
  
• click open creative mail, this should open the creative mail plugin in a new tab.

[jetpackbot]

	Warnings
⚠️	The PR is missing at least one [Status] label. Suggestions: [Status] In Progress, [Status] Needs Review
⚠️	"Proposed changelog entry" is missing for this PR. Please include any meaningful changes

This is an automated check which relies on PULL_REQUEST_TEMPLATE. We encourage you to follow that template as it helps Jetpack maintainers do their job. If you think 'Testing instructions' or 'Proposed changelog entry' are not needed for your PR - please explain why you think so. Thanks for cooperation 🤖

E2E results is available here (for debugging purposes): https://jetpack-e2e-dashboard.herokuapp.com/pr-17029

Generated by 🚫 dangerJS against 2134cb2

[jeherve]

This looks good to me. Merging.

[jeherve]

Cherry-picked to branch-8.9 in 7ed3798

[jeherve]

Internal reference: r212966-wpcom