ENH: Highlight primer mutations in sequence view similar to indels, with a marker, e.g. an `x`

[corneliusroemer]

Matthijs Welkers made a good suggestion to make the primer feature more useful. Right now primer mutations are not surfaced in sequence view. We could surface them similar to how we started surfacing indels with little markers.

This could be turned off by users (or be off by default) for users that don't care about primer mutations.