Employee Engagement basics: Create code.org/company-profiles/* landing page

Create code.org/company-profiles/* landing page with default text that is customized with the company name. Add create-company-profile and edit-company-profile forms to allow companies to edit name, text (markdown), video, and logo. Store company name in activity table and retrieve company name on the congrats certificate page.

These pages are not being publicized yet and there is lots of fit and finish still to do.
we need a "user_entered_markdown" helper that doesn't allow embedded HTML. it should just be a flag in RedCarpet to always encode < and > tags.