Support showing predictions for future dates in the dashboard

Allow users to request and see predictions for future dates made by selected forecasters.

Makefile: pull_data target to pull new saved datetime.
app/R/data.R: data loader to get cached datetime in addition to scores.
app.ui.R: enable "show forecasts" button for all as-of dates by default.
app/R/data_manipulation.R: filterOverAllLocations() to take new filterDate arg. filterDate is used to remove predictions that are in the past (relative to when the data was generated) but don't have scores despite having truth data available. This lets us keep predictions that are in the future and don't have scores (because truth data wasn't available).
app/server.R:
- Get new cached datetime.
- Explicitly set summary plot and truth plot x-axis ranges to be the same when predictions are shown on the truth plot. Since predictions are shown only on the truth plot and can appear in the future, axis auto-scaling can extend the max date past what is displayed in the summary plot and make it hard to compare between the plots.
- Patch the truth plot legend parsing. The plotly package seems to have changed handling of ggplot legends so that we see duplicate entries for a given forecaster.
- Change filterScoreDf() logic to match `filterOverAllLocations()
- Generalize filterForecastData() to work with (past as-of dates) or without (most recent as-of date) as-of data.
- Remove blocks to show or hide the "show forecasts" button, since we want to show it for all as-of dates now.

Closes #203

cmu-delphi / forecast-eval