Added Help dropdown menu to banner

This is for issue 380 https://github.com/hotosm/osm-tasking-manager2/issues/380

1) Added a help_menu.mako template for a help dropdown menu. It has the links to the websites (OSM Wiki, LearnOSM, and MapGive) that were originally on the About page. Eventually there might be a link to a Tasking Manager Quick Start guide here.
2) Deleted the Getting Started paragraph in the About page that had the links.
3) Deleted the About link the footer -- it seemed redundant.
4) Before I changed the dropdown menu's class to "dropdown-menu" instead of "dropdown-menu languages", I thought I had to add new msgid and msgstr statements for "OSM Wiki", "LearnOSM", and "MapGive" in all of the osmtm.po files. Which is why the .idea files are changed. I deleted the changes I made to the osmtm.po files and didn't push them to my fork.