Improve selection of TPMAs

[matt-dray]

Problem

At the moment we present one long list of mitigators in a dropdown list. It is searchable and includes the code as well, but we should make it easier for people to find the TPMA they're looking for.

UI

The simplest solution is to group TPMAs within the dropdown itself, which can be achieved with a named list, e.g. the example in the docs:

    selectInput("state", "Choose a state:",
      list(`East Coast` = list("NY", "NJ", "CT"),
           `West Coast` = list("WA", "OR", "CA"),
           `Midwest` = list("MN", "WI", "IA"))
    )

That might be better in the (very) short-term.

A more complex solution is what we have in nhp_compare_mitigation_predictions_app (see the deployment if you have sufficient permissions). It uses datamods::select_group_*() to provide a box of dropdowns where the user can filter by certain groups and then confirm their selection with a button. But we had a small issue with that.

Groups

How should we group the TPMAs? As per nhp_compare_mitigation_predictions_app? What do users want to see?