Regenerate Existing Images Job: Create main settings screen

[mxbclang]

Create a admin settings screen to start the Regenerate Existing Images background job. This will be a new screen available under the 'Tools' menu and display basic settings to start the Regenerate Existing Images background job.

Acceptance Criteria

• Create a new admin settings screen under “Tools > Regenerate Images”
• This screen is only accessible to users with the upload_files and manage_options capabilities.
• The screen will display the following on the page
  • The title "Regenerate Existing Images"
  • A brief description explaining the feature
  • A basic form containing the following controls:
    • A "Regenerate Images" button that will start the Regenerate Existing Images background job.
    • A delete_images checkbox to delete old images. By default the checkbox is unchecked.
• When the form is submitted, the POST request is handled by doing the following:
  • Validate the form submission with wp_verify_nonce
  • Check the delete_images checkbox value
  • Create a new background job with the perflab_create_background_job function passing the following
    • regenerate_images as the job name
    • array( 'delete_images' => $delete_images ) as the job data, passing the value of the delete_images checkbox as part of the data
  • Using the $job_id returned by perflab_start_background_job function
    • Passing the newly created $job_id returned by the perflab_create_background_job function