Added markdown syntax cheat sheet link #221

This is my first stab at adding a markdown syntax cheat sheet link to
help people compose their posts (issue #221). The link opens a github
hosted markdown guide in a new tab.
Merge branch 'master' into mkdown-cheat-sheet-link
Added markdown link to new thread view #236

Added markdown help link to new thread view (added it to new post view
in previous commit). Refactored CSS by moving relevant rules to from
thread to post.css.scss, and placed rules in scope of new post and new
thread markdown. Changed the text from "Parsed as Markdown" to
"Markdown cheat sheet"
Move duplicated markdown help markup to a partial.
Fix markdown help mobile styles and edit view.
Expand definition of `open-in-new-window?` to know about middle clicking.
Break long words in posts (e.g. links with long components)
Add "Issues" link to footer. Change "Source" to "Code".
Merge branch 'handle-middle-click'

Conflicts:
	app/assets/javascripts/client-prod.js
Merge branch 'word-break'
Merge branch 'link-to-issue-tracker'

Conflicts:
	app/assets/javascripts/client-prod.js
Recompiled client-prod.js