Public summaries and sharing links

[muety]

Allow a user to make her summary public, so a read-only view of it can be shared as a link on the web.

The user should able to choose which charts to display on the public page and which to hide.

[muety]

Inspired by WakaTime embeddable charts.

[muety]

I'd suggest to do it analogously to WakaTime, which has two checkmarks to enable exposing statistics publicly.

These two settings will affect the stats endpoint (#114) as well as the badge endpoint.

To Do

• Add two new flags to the user
  • int flag indicating how many days to share publicly (default 0)
  • Complementing bool flag indicating whether to include projects, languages, etc. in publicly shared stats
• Substitute badges_enabled flag with the two from above and write according database migration