Fixes #1912: Add "Loading..." messages to slow functions within Annual Peak Streamflow plot

[maggiejaenicke]

• Adds loading messages for:
  • chooseFloodStats()
  • choosePeakYear()
  • dateRangePicker()
• Adds a commented loading message for the functions in afterSetExtremes(), but left it commented because I don't think it looks very good, but you can experiment with turning those on and off if you want to see. (GagePageController.ts lines 2465-2476).
• Fixes a minor issue with labels appearing for flood stats (UTC issue)
• Fixes a time zone issue (see Andrea's comment). 03138500 has no Instantaneous Flow so an error was causing plot not to load.

Closes #1912

[amedenblik]

Deployed to https://dev.streamstats.usgs.gov/gageplots/