refactor: track different kinds of tables in the Python script

Define an enumeration of all kinds of tables this script generates.
Determine the PR title and description from this kind; this means we may remove the corresponding entries in the bash script.
To prevent the .sh and .py script from getting out of sync, complain if any unexpected file was passed to the Python script.

This sets the ground-work for #16: mapping files to PR tables will allow merging json files on the Python side.

jcommelin / queueboard