Tweet prompt upon publishing

Note that our old twitter link on view post was broken due to Twitter
not accepting hashtags in urls. I fixed it with a unicode value for #
instead, and made a single variable to use in both places. The value of
the link now includes title and tags of the post as well
trigger twitter prompt when article is created
Syntax correction and tweeting in new tab
Added missing '/' in tweet link
Merge pull request #5 from newsquid/tweet_prompt

Tweet prompt