Show TS version & changelog

[jkppr]

Is your feature request related to a problem? Please describe.
As a user, I would like to know:

• What version of Timesketch I'm using
• If an upgrade/downgrade has happened
• What changed (new features and fixes)

Describe the solution you'd like
Let curious users see the version number somewhere, maybe not on every page but perhaps an "About Timesketch" dialog or a corner of an existing page.

Notify users when an upgrade has occurred, maybe include a change log to socialize all the hard work we've done.