Add summaries for notes and merge requests to dashboard feed.

This fixes #305.
Updated ATOM feed partials to use markdown instead of simple_format.
Remove stale comment in Gemfile

The comment was added in a91a6491bf3e74791ecf8dd1111d54435b8578cb, and referred to the below gem `grape-entity` being pulled directly from a Git repo. The `gem` command has already been changed to pull directly from Rubygems, so the comment is no longer necessary.
Merge branch 'fix-stale-gemfile-comment' into 'master'

Remove stale comment in Gemfile

The comment was added in a91a6491bf3e74791ecf8dd1111d54435b8578cb, and referred to the below gem `grape-entity` being pulled directly from a Git repo. The `gem` command has already been changed to pull directly from Rubygems, so the comment is no longer necessary.
Merge branch 'bugfix/305/dashboard_feed_summaries' into 'master'

Incorporate Summaries for Notes and Merge Requests Into Dashboard Feed

This MR fixes #305 by adding summaries for notes and merge requests into the dashboard feed.