Cannot find "pH" in characteristic filter on import tab

[cristinamullin]

Describe the bug

It is difficult/impossible for users to query for "pH" data because they cannot find "pH" in the characteristic drop down in the Shiny app.

To Reproduce

Steps to reproduce the behavior:

1. Open the shiny app

2. On the import tab, under Option B, enter "pH" into the Characteristic(s) query box. The app will filter all characteristics to those that include "ph" which is a VERY long list of characteristics and include that below. However, a user cannot actually find "pH" in this list, and the user is not able to simply enter "pH" as text in the box.

library(TADAShiny)
TADAShiny::run_app()

Expected behavior

Maybe we can get the drop down in the shiny app to order the characteristics, so the most common/priority ones (including pH) show up at the top of the drop down list. This is probably the best option.

And/or make it so a user can simply enter text into the characteristic box on the shiny app without having to select from the drop down. However this second option is problematic if the user enters text that does not correspond to a specific WQX characteristic - we would need to throw a warning in that case & make sure that scenario does not crash the app.

Note:
Here is a link to the WQX domain table that includes all characteristics (this drives the characteristic drop down in TADAShiny): https://cdx.epa.gov/wqx/download/DomainValues/Characteristic.CSV

pH is a valid characteristic included in the list.