Add sharezone_lints to app_functions

[nilsreichardt]

This PR adds sharezone_lints to the app_functions package and fixes the following lint warnings:

Analyzing app_functions...                                              

   info • Don't invoke 'print' in production code • lib/src/app_functions.dart:28:7 • avoid_print
   info • Don't invoke 'print' in production code • lib/src/app_functions.dart:46:5 • avoid_print
   info • Use interpolation to compose strings and values • lib/src/app_functions.dart:46:11 • prefer_interpolation_to_compose_strings
   info • Don't invoke 'print' in production code • lib/src/app_functions.dart:47:5 • avoid_print
   info • Use interpolation to compose strings and values • lib/src/app_functions.dart:47:11 • prefer_interpolation_to_compose_strings
   info • Don't invoke 'print' in production code • lib/src/app_functions.dart:48:5 • avoid_print
   info • Statements in an if should be enclosed in a block • lib/src/app_functions.dart:50:7 • curly_braces_in_flow_control_structures
   info • Statements in an if should be enclosed in a block • lib/src/app_functions.dart:58:7 • curly_braces_in_flow_control_structures
   info • Don't invoke 'print' in production code • lib/src/app_functions.dart:59:5 • avoid_print
   info • Use interpolation to compose strings and values • lib/src/app_functions.dart:59:11 • prefer_interpolation_to_compose_strings
   info • Don't invoke 'print' in production code • lib/src/app_functions.dart:60:5 • avoid_print

Part of #37

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