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-local.sh

@@ -12,7 +12,7 @@
 # LOCAL_ANSWERS (kept in a variable because $(...) would drop the last newline).
 local_answers() {
   if [[ $2 == y ]]; then
-    printf -v LOCAL_ANSWERS 'my-app\nA test app\n\nTirSystem\ny\nacme-org\n%s\n%s\n' "$1" "$3"
+    printf -v LOCAL_ANSWERS 'my-app\nA test app\npublic\nTirSystem\ny\nacme-org\n%s\n%s\n' "$1" "$3"
   else
     printf -v LOCAL_ANSWERS 'my-app\n\n\nTirSystem\nn\n%s\n%s\n' "$1" "$3"
   fi