improve indicator dashboard indicator/signal names

[sgratzl]

coming from cmu-delphi/www-covidcast#931 (comment):

The indicator names should be more descriptive.

[krivard]

Yeah we need to figure out what the indicator status "items" actually represent -- they started out as "representative signal for this source" but that makes more sense for coverage data than for backfill profiles.

[sgratzl]

see also https://docs.google.com/document/d/1KVRezwvEPM2qG1nSolTiPXu6R_LoaF3YgG5en2glcM8/edit#

[sgratzl]

superseded by #559 -> closing