Save translation report information in a local file with a timestamp

Our translation report page shows the percentage of strings translated for each sim. The information needed to construct this page is gathered by the server through GitHub requests and analysis of the sim HTML files. This information is cached in local memory for the process, and has to be re-compiled anytime the server is restarted. When doing a lot of debugging, this can lead to a situation where GitHub decides to throttle access, see https://github.com/phetsims/rosetta/issues/410. This affects not only the version being locally tested, but also the live version, which is bad. Not to mention that it's annoyingly slow to have to rebuild these reports after every restart.

I think I should investigate caching the information in a locally stored file instead. This would lead way fewer GitHub requests when doing a bunch of restarts and save time during debugging since the reports wouldn't need to be rebuilt after every restart.