Improve LICENSE + copyright information & CONTRIBUTING file

[Ana06]

• Replace LICENSE text file with the official Apache one: https://www.apache.org/licenses/LICENSE-2.0.txt. This changes Copyright (C) 2021 Mandiant, Inc. by the following template language that had been incorrectly replaced: Copyright [yyyy] [name of copyright owner]

• Improve CLA information to adhere to Google policies (one needed sentence was missing).

• Use Google's default Code of Conduct for consistency with other Google's projects.

• Replace the header from source code files. The old header had the confusing sentence All rights reserved, which does not make sense for an open source license. Replace the header by the default Google header that corrects this issue and keep capa consistent with other Google projects. Also replace also the copyright text in the web/public/index.html file for consistency and adapt the linter to work with the new header. The changes in the copyright header have been made with the following script:

  import os
  import re
  
  OLD_HEADER = b"# Copyright \(C\) (?P<year>\d+).* Mandiant, Inc(.|\n)*See the License for the specific language governing permissions and limitations under the License.\n"
  NEW_HEADER = b"""# Copyright %s Google LLC 
  #
  # Licensed under the Apache License, Version 2.0 (the "License");
  # you may not use this file except in compliance with the License.
  # You may obtain a copy of the License at
  #
  #     http://www.apache.org/licenses/LICENSE-2.0
  #
  # Unless required by applicable law or agreed to in writing, software
  # distributed under the License is distributed on an "AS IS" BASIS,
  # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  # See the License for the specific language governing permissions and 
  # limitations under the License.
  
  """ 
  
  for dir_path, dir_names, file_names in os.walk("capa"):
      for file_name in file_names:
          # header are only in `.py` and `.toml` files
          if file_name[-3:] not in (".py", "oml"):
              continue
          file_path = f"{dir_path}/{file_name}"
          f = open(file_path, "rb+")
          content = f.read()
          m = re.search(OLD_HEADER, content)
          if not m:
              continue
          print(f"{file_path}: {m.group('year')}")
          content = content.replace(m.group(0), NEW_HEADER % m.group("year"))
          f.seek(0)
          f.write(content)

  Some files had the copyright headers inside a """ comment and needed manual changes before applying the script. hook-vivisect.py and pyinstaller.spec didn't include the license in the header and also needed manual changes.

• Add missing headers to the source code files inside the web directory and the capa/render/proto/capa.proto file. I have used addlicense to add the headers. I think it would be a good idea to use addlicense as linter in this project, but I think we should wait until the following issues are resolved (and ensure there are no other issues): Ignore empty files google/addlicense#162, Support spec files google/addlicense#163. I can open an issue after this PR has been merged to propose using addlicense as linter.

Related PRs in other Mandiant repositories: mandiant/flare-vm#638, mandiant/VM-Packages#1238

Checklist

• No CHANGELOG update needed

• No new tests needed

• No documentation update needed

[williballenthin]

lgtm, thanks @Ana06!

[williballenthin]

two things:

• a few small formatting errors called out by black
• please recognize yourself in the changelog