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/lib.sh

@@ -21,6 +21,16 @@ readonly FAKE_GITHUB_PAT="ghpFAKEtoken1234567890"
 # Answers to the prompts: name, description, visibility, Gitea owner, GitHub
 # yes or no, GitHub owner, directory, plan gate.
 readonly ANSWERS_GITHUB=$'my-app\nA test app\n\nTirSystem\ny\nacme-org\n\nn\n'
+# The same, for a public project (the AGPL-3.0 default applies with GitHub).
+readonly ANSWERS_GITHUB_PUBLIC=$'my-app
+A test app
+public
+TirSystem
+y
+acme-org
+
+n
+'
 readonly ANSWERS_GITEA_ONLY=$'my-app\n\n\nTirSystem\nn\n\nn\n'
 
 SHARED_REMOTES=""