Provide backup data if loading from S3 is slow

Occasionally it takes a _really_ long time (1 min vs the normal 15-20s) to load input data from the S3 bucket. This problem can be seen in both staging and public dashboards at the same time, which indicates that it is an AWS or network problem rather than a bug in our code. Can [load fallback data](https://github.com/cmu-delphi/forecast-eval/blob/dev/app/R/data.R#L54) if directly loading from AWS takes longer than [x] seconds.

Start in app/R/data.R around line 54, where input data is loaded directly from AWS. Trace the loading path and determine how the fallback data is currently made available. Done means slow AWS loads use fallback data after an agreed timeout while normal loads continue to work.

Written by the indexing model from the issue text.