shorten per-file copyright headers

I'm tired of all the boilerplate, so use the new
GPL-3.0-linking-exception license identifier instead in all the server
components.

I left the ui stuff alone because I'm just going to replace it (#111).

Add a checker for the header because it's easy to forget.

.github/workflows/check-license.py

@@ -0,0 +1,68 @@
+#!/usr/bin/env python3
+# This file is part of Moonfire NVR, a security camera network video recorder.
+# Copyright (C) 2021 The Moonfire NVR Authors; see AUTHORS and LICENSE.txt.
+# SPDX-License-Identifier: GPL-v3.0-or-later WITH GPL-3.0-linking-exception.
+"""Checks that expected header lines are present.
+
+Call in either of two modes:
+
+has-license.py FILE [...]
+    check if all files with certain extensions have expected lines.
+    This is useful in a CI action.
+
+has-license.py
+    check if stdin has expected lines.
+    This is useful in a pre-commit hook, as in
+    git-format-staged --no-write --formatter '.../has-license.py' '*.rs'
+"""
+import re
+import sys
+
+# Filenames matching this regexp are expected to have the header lines.
+FILENAME_MATCHER = re.compile(r'.*\.(py|rs|sh|sql)$')
+
+MAX_LINE_COUNT = 10
+
+EXPECTED_LINES = [
+  re.compile(r'This file is part of Moonfire NVR, a security camera network video recorder\.'),
+  re.compile(r'Copyright \(C\) 20\d{2} The Moonfire NVR Authors; see AUTHORS and LICENSE\.txt\.'),
+  re.compile(r'SPDX-License-Identifier: GPL-v3\.0-or-later WITH GPL-3\.0-linking-exception\.'),
+]
+
+def has_license(f):
+  """Returns if all of EXPECTED_LINES are present within the first
+  MAX_LINE_COUNT lines of f."""
+  needed = set(EXPECTED_LINES)
+  i = 0
+  for line in f:
+    if i == 10:
+      break
+    i += 1
+    for e in needed:
+      if e.search(line):
+        needed.remove(e)
+        break
+    if not needed:
+      return True
+  return False
+
+
+def file_has_license(filename):
+  with open(filename, 'r') as f:
+    return has_license(f)
+    
+
+def main(args):
+  if not args:
+    sys.exit(0 if has_license(sys.stdin) else 1)
+
+  missing = [f for f in args
+             if FILENAME_MATCHER.match(f) and not file_has_license(f)]
+  if missing:
+    print('The following files are missing expected copyright/license headers:')
+    print('\n'.join(missing))
+    sys.exit(1)
+
+
+if __name__ == '__main__':
+  main(sys.argv[1:])

.github/workflows/ci.yml

@@ -60,3 +60,10 @@ jobs:
     - run: cd ui && npm ci
     - run: cd ui && npm run build
     - run: cd ui && npm run lint
+  license:
+    name: Check copyright/license headers
+    runs-on: ubuntu-20.04
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v2
+    - run: find . -type f -print0 | xargs -0 .github/workflows/check-license.py