Set the project license in config.env; AGPL-3.0 default only for a public GitHub project

- PROJECT_LICENSE (a Gitea license key, or none) is read, checked and never asked
- Without it AGPL-3.0 applies only when GitHub is chosen and the project is public
- The license is applied on Gitea with or without GitHub, and checked against the server first
- Plan and summary name the license and where it came from
- MIL-006 accepted; README and config.env.example document the key
- Tests: new test-license.sh; existing tests use a public project where they expect AGPL-3.0

Task: MIL-006#1
Task: MIL-006#2
Task: MIL-006#3
Task: MIL-006#4
Refs #44
Refs #45
Refs #46
Refs #50

tests/test-presets.sh

@@ -205,7 +205,7 @@ test_summary_marks_the_values_from_config_env() {
   assert_contains "name" "$OUT" "Repository   : my-app (from config.env) (private (from config.env))"
   assert_contains "description" "$OUT" "Description  : A test app (from config.env)"
   assert_contains "Gitea" "$OUT" "/TirSystem/my-app (from config.env)"
-  assert_contains "GitHub" "$OUT" "(AGPL license applied) (from config.env)"
+  assert_contains "GitHub" "$OUT" "/acme-org/my-app (from config.env)"
   assert_contains "directory" "$OUT" "Directory    : ./my-app (from config.env)"
   assert_contains "plan gate" "$OUT" "Plan gate    : no (from config.env)"
 }