This PR alters the preview record selector pager present in
the mass_mailing mobile preview view to allow users to manually
select a recipient, instead of rotating through the available
recipients.

This PR also introduces an alternate-color version of the
HTML editor phone_svg image, to be used in the dark theme preview.

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This PR introduces a series of small fixes to the mailing form view,
UX and associated views:

- When clicking its Discard button, the fullscreen mailing editor only
  discards changes made to the mailing
- In the Mailing preview dialog, the "fake" header no longer displays
  a grab cursor (as it cannot be grabbed)
- When sending a test, a toaster is no longer displayed, as information
  pertaining to its success or failure is already logged in the chatter
- The Dynamic list recipient count is no longer displayed for sent
  mailings, as it may have become outdated since it was sent.

This commit also removes the outdated demo in mass_mailing_sale. It
was no longer compatible with the editor, and its previous purpose
in demonstrating link tracking is now handled by other demo mailings.

task-6321624