Support Twitter Hashtags for Sessions

[bcholmes]

(Creating an issue based on a Slack conversation).

In some cons, the programming team encourages folks to engage with sessions using Twitter (where audience -- or panelists -- can opine about the content of a session separate from raising questions in the room) or other social media. To make it easier to find a Twitter conversation, the con will propose a recommended hash tag for each session.

This improvement request proposes adding a field to the sessions for recording a hashtag. Making hashtag a separate field (rather than, say, embedding it in the description) allows us to extract and use the hashtag in a few ways such as:

• printing a "hash tag" table tent
• having specialized buttons in the mobile app that can launch the "compose a tweet" with the hashtag pre-filled.
• and so forth.

We probably want to add some additional hashtag-related features: setting a default hashtag (based on the session title?), reports for overly-long hashtags, etc.