Add licensing lines to all the code files

[IllyaMoskvin]

Many files are missing the requisite licensing declaration @hburgund mentioned in #283. I think it makes sense to treat this as a separate issue, go through all the files, and create a separate pull request.

# Roundware Server is released under the GNU Affero General Public License v3.
# See COPYRIGHT.txt, AUTHORS.txt, and LICENSE.txt in the project root directory.

[hburgund]

Good idea to do a full audit of all files, but please still add the lines to the new files you create in #283 so they are properly licensed right from the start.

[13rac1]

A few py files missing the AGPL notice:

• https://github.com/roundware/roundware-server/blob/db3a02d41415ab8f39c41a0c7d0afd031a2c4c8c/roundware/rw/mommy_recipes.py
• https://github.com/roundware/roundware-server/blob/db3a02d41415ab8f39c41a0c7d0afd031a2c4c8c/roundware/notifications/admin.py
• https://github.com/roundware/roundware-server/blob/db3a02d41415ab8f39c41a0c7d0afd031a2c4c8c/roundware/notifications/admin.py
• https://github.com/roundware/roundware-server/blob/db3a02d41415ab8f39c41a0c7d0afd031a2c4c8c/roundware/lib/api.py
• https://github.com/roundware/roundware-server/blob/db3a02d41415ab8f39c41a0c7d0afd031a2c4c8c/roundware/api2/filters.py