Adds open source licenses (fixes #2941)

[alexzvk]

Description

My EECS 481 final project. Takes the pull request from @Andrew-Chen-Wang (who I have been in contact with) that fixes #2941 and implements suggestions on said pull request.

• Adds a script check_licenses.py which iterates through all licenses and prints any instances of placeholders for cookie-cutter project data that have not been reformatted to accept said project data.
• Adjusts update_licenses.py to store a mapping of license names to file titles to a dictionary, found in a json file in the project slug's "licenses" folder. This reduces the time complexity of the previous solution, which involved looping through all of the possible licenses, and also removes the need to include Jekyll headings (though they have not been removed)

Note that the build works locally for me, but I was not able to pass all of the automated checks. While there remains much I want to work on... the end of the semester looms and I must make a pull request to receive credit.

Checklist:

• I've made sure that tests are updated accordingly (especially if adding or updating a template option)
• I've updated the documentation or confirm that my change doesn't require any updates
  In line with the previous pull request: not sure if it's worth putting all of the licenses in the documentation.

Rationale

Fixes #2941
Makes it easier to maintain having many options for open-source licenses since a script can be run that pulls them all from a maintained github repository