Display archived signals in separate tab on dashboard

[nmdefries]

Description

Since JHU has been discontinued, forecasts using it as truth data (for plotting and for scoring) are no longer supported by ForecastHub. Forecasts for cases and deaths are moving to "archival" status and should be differentiated from the active target variable hospital admissions.

To do so, cases and deaths forecasts will be displayed in a separate tab on the dashboard. Since we want to have the new tab use the same/existing format as the "current" tab, the data handling and plotting logic will be shared.

IDs of objects in the dashboard (plots, text, buttons) must be unique, so to reuse the code in a general way, modify interactive object IDs by appending a tab-dependent suffix. Add reactivity to support this.

Note:

• At the moment, this new tabbed dashboard fetches all the data on load. This will be improved in an upcoming PR.
• The separate-dashboards version of this is working at commit fe4f6b7.

Changes

• Makefile -- set working dir for easier local runs
• data.R -- simplify data load
• server.R -- observe tab change; send results to tab-appropriate output ids
• ui.R -- factor out plot area layout and reuse for "archive" tab
• style.css -- apply the same formatting to the "archive" tab

Fixes

Partially addresses https://github.com/cmu-delphi/forecast-eval/issues/175

[nmdefries]

~Tests are fixed on another branch, will merge that in when available. The issue is with the GH action and not with the code here, so the dashboard behavior won't be modified.~ This is resolved.

[nmdefries]

Closes https://github.com/cmu-delphi/forecast-eval/issues/250