Date handling improvements

[acelaya]

• Allow the default timezone used by Shlink to be customized, ideally, via env vars. #1418
  • Document the recommendation to set Shlink on the same timezone it's going to be primarily consumed via API.
• Parse provided dates to the instance timezone, before trying to use them in a database query. #1419
  This assumes all visits in the database have happened in current system timezone. That could be false if the timezone is changed later in time, but that's an acceptable edge case for now.