Adds step-by-step instructions for mobile app users including the
corresponding image icons for the buttons that they should tap on
their screen.
Makes individual SVG image icons, for the help center docs, available
in `/static/images/help/`, instead of importing the entire set of
icons as dependencies.
Adds the icons for the "PMs" tab from https://feathericons.com, and
"Checkmark" and "Send" buttons from https://materialdesignicons.com.
Adds a new nested CSS selector to `.markdown img` so that the default
white border for images does not get applied to these icons.
David Rosa