Remove obsolete determination of hostname.
Clarify that connection string is not necessarily an URL.
Add DB class and move database operations into it.
Add YouDo.DB.Mock with trivial in-memory implementation.
Use Options.Applicative to parse command line.
Add option to run with transient in-memory database.
Remove obsolete command-line option stuff.
Merge branch 'dbclass'

* dbclass:
  Remove obsolete command-line option stuff.
  Add option to run with transient in-memory database.
  Use Options.Applicative to parse command line.
  Add YouDo.DB.Mock with trivial in-memory implementation.
  Add DB class and move database operations into it.
Move content of server.hs to module YouDo.Web