Update Shiny app

- Remove template/logic for single data entry
- Change 'Add estimators' to 'About estimators'; adding estimators and viewing estimates are now done in the same tab
- Swap rows and columns in estimates table (estimators as row names, datasets as column names)
- Add a separate column for the serial interval to the estimates table
- Add plots for daily and weekly data using the Plotly library
- Remove italics from the subscript in all occurences of 'R_0'
- Fix code/text formatting