[devtools] script support for managing source file copyright headers

Three subcommands to this script:

1. ./copyright_header.py report

Examines git-tracked files with extensions that match:

INCLUDE = ['*.h', '*.cpp', '*.cc', '*.c', '*.py']

Helps to:

-> Identify source files without copyright
-> Identify source files added with something other than "The Bitcoin Core
developers" holder so we can be sure it is appropriate
-> Identify unintentional typos in the copyright line

2. ./copyright_header.py update

Replaces fix-copyright-headers.py. It does file editing in native python
rather than subprocessing out to perl as was the case with
fix-copyright-headers.py. It also shares code with the 'report' functions.

3. ./copyright_header.py insert

Inserts a copyright header into a source file with the proper format and
dates.