Add a new method for activity_header that determines the heading level
depending on whether the theme displays a heading for the activity
header (usually a h2 heading with the activity name).
E.g. in Boost, the activity name is already being displayed in a
heading. So page headings can be rendered as h2. However, on Classic,
the activity name is being displayed as a h2 heading. So headings need
to be adjusted for the activity pages.
Jun Pataleta