Fixes broken Images on private sites in local preview

[marecar3]

Fixes # #8964

Initial problem was that EditPreviewPostFragment wasn't using WPWebViewClient as a client in WebView which exposed an issue #8964. (Missing logic for inserting the token when fetching private site image)

To test:

Steps to reproduce the behavior

Test A:

1. Select a WordPress.com Simple site and set it to Private.
2. Start a new post.
3. Add an image to the post. (Wait for it to upload or select an image from the site's media library.)
4. Preview the post. Result: The local draft shows an image.
5. Back to the post, add some content
6. Preview the post. Result: It should have the latest changes
7. Rotate device. Result: Content should be there

Update release notes:

• If there are user-facing changes, I have added an item to RELEASE-NOTES.txt.
  xt`.

[mzorz]

Should the base branch be release/12.1 @marecar3 ?

[mzorz]

Tested the scenarios, enabled Don't keep activities and rotated, etc and works alright 👍
Code LGTM as well, so