Add about dialog with license info
Move AboutDialog to correct path
Use real copyright symbol ©
Convert about dialog to activity
link to account details when touching own avatar
Make links clickable in about text
Use normal text size and color
Shorten license text and fix height of scrollview
Add last seen info to the database

Fixes #612
Merge pull request #616 from SamWhited/license-compliance

License compliance via an "about" menu item
Merge branch 'development' of https://github.com/siacs/Conversations into development
Merge pull request #626 from SamWhited/issue612

Add last seen info to the database