Create endpoint to post notices from host apps

[chipsnyder]

Fixes #2453

Related PRs:
gutenberg WordPress/gutenberg#25266
WordPress-iOS wordpress-mobile/WordPress-iOS#14886

To test:

🗒️ Test cases here are taken from wordpress-mobile/WordPress-iOS#14886

To disable or enable the development version of Modal Layout Picker

• Open the app from the build that allows FeatureFlags such as a PR build or a local development build
  • By default, the modal layout picker will be disabled.
  • From the site page:
    • Click on your Gravatar.
    • Click on App Settings.
    • Click on Debug.
    • Toggle "Gutenberg Modal Layout Picker" to enable or disable the picker
      

When enabled, the Layout Picker should show when creating a new page from My Site and from the Page list.

Start by navigating to the Modal Layout Picker

Page Created (with content)

1. Select a Layout from the options in Modal Layout Picker
2. Select "Create Page"
   Expect to see the info notice banner appear under the navigation bar with the text "Page created"

• 🗒️ This is different than the mocks in the linked issue but aligns with the direction that was determined.

Blank Page Created

1. Select "Create Blank Page"
   Expect to see the info notice banner appear under the navigation bar with the text "Blank page created"

• 🗒️ This is different than the mocks in the linked issue but aligns with the direction that was determined.

Blog Post Created

1. Create a new blog post

• This should not display the notice bar

Open existing Page Created

1. Open an existing page (either published or draft)

• This should not display the notice bar

Disable Modal Layout Picker

1. Create a new page

• This should not display the notice bar

Screenshots:

Create Page	Create Blank Page	Clip

Additional Context:

This is a continuation of the work done as part of the Modal Layout Picker Project and will be followed up by other tickets as part of Modal Layout Picker Project

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered if this change warrants user-facing release notes more info and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

Wanna run full suite of Android and iOS UI tests? Click here and 'Approve' CI job!

[geriux]

Approved via WordPress/gutenberg#25266 (review)