Fix preview links and button for emails

[merkushin]

Resolves #7458

In the issue it is said, that "View Email" link is broken, but when I started testing, other places had also incorrect link.

There were changes in Gutenberg that broke our preview buttons:

• For example, changes in this PR made our EmailPreviewButton not working: Could we just delete this component and all related code?
  (I'm still looking for information about it more.)
• Here is the change for the View post link, that was originally introduced here and later moved to this place with a new name. (Not sure about underlying changes.)

Proposed Changes

• Filter the preview link.
• Remove email-preview-button component.

Testing Instructions

1. Go to Sensei LMS -> Settings -> Emails
2. Open one of the email in the editor.
3. Make sure the preview button in the top bar works, and the URL in the sidebar is also correct and works.
4. Make a change in the email content and click "Update".
5. The notice with text "View Email" appears.
6. Click on "View Email" and make sure you see the email preview.
7. Go to Posts/Pages/Courses and makes sure their View/Preview links work properly.

Pre-Merge Checklist

• PR title and description contain sufficient detail and accurately describe the changes
• Acceptance criteria is met
• Decisions are publicly documented
• Adheres to coding standards (PHP, JavaScript, CSS, HTML)
• All strings are translatable (without concatenation, handles plurals)
• Follows our naming conventions (P6rkRX-4oA-p2)
• Hooks (p6rkRX-1uS-p2) and functions are documented
• New UIs are responsive and use a mobile-first approach
• New UIs match the designs
• Different user privileges (admin, teacher, subscriber) are tested as appropriate
• Legacy courses (course without blocks) are tested
• Code is tested on the minimum supported PHP and WordPress versions
• User interface changes have been tested on the latest versions of Chrome, Firefox and Safari
• "Needs Documentation" label is added if this change requires updates to documentation
• Known issues are created as new GitHub issues

[github-actions]

Test the previous changes of this PR with WordPress Playground.

[github-actions]

Test the previous changes of this PR with WordPress Playground.

[m1r0]

Is this ready for review?

[github-actions]

Test the previous changes of this PR with WordPress Playground.

[github-actions]

Test the previous changes of this PR with WordPress Playground.

[github-actions]

Test the previous changes of this PR with WordPress Playground.

[donnapep]

Works great!