Add more env vars & unify show-dms

[volker-fr]

1. Add a few more environment variables where they have been missing
2. All new environment variables added in the last months start now with SEV_
3. unify the cli's and webservers command line flag to show/hide DM's.
4. add a new flag to not add the "Thread message:" to a message. Default is to keep "Thread message:" to be backward compatible. The use case is not really clear at this point since threads can now be identified differently. In the past the template did a check, but this has been improved in Improve thread representation #204
5. Update --help documentation to contain the environment variable & default value.
6. Remove the click helper utils file after replacing it with the click internal env var & default value option.

(2) and (3) are not backward compatible changes. Since these flags are relatively new, having a breaking change here should be fine. Click will also warn about a not supported flag.