Use "item" terminology consistently in the UI

Most of the user interface refers to events and to-dos as "items", not
"instances".  For example, look at the dialog that "Archive Old Incidences..."
opens.

GUI:
1 job for incidence-item in 12 minutes and 44 seconds (queued for 53 minutes and 18 seconds)
Status Job ID Name Coverage
  Build
passed #92863
build_clazy_clang_tidy

00:12:44