New user onboarding

[raccube]

When a new user logs in for the first time, they are immediately presented with an empty feed. It would be much better if they were instead presented with some information about how they can use the site.

Some possible points:

• Follow your friends. [Some text explaining how sending questions to followers works]
• Customise your theme.
• Change your settings. [Some text explaining privacy settings, etc.]
• Share automatically to Twitter.

[pixeldesu]

I would like the onboarding to be a bit more organic. So, actually placed that info is shown where relevant and so it's kind of like a gradual discovery (if you use the site).

• Empty feed: Would be cool to have a profile discovery page, to get people you could follow, pretty much.
• First time opening inbox: Explaining social share, and the muting options.
• Empty inbox: Large callout for social share buttons or generating questions.
• Profile with theme, logged in user not having a theme: Showing a notification inline about this user having a themed profile and linking to the theme editor.
• Question modal without followers: #211