Replace "web application" with "web app"

#1875 (closed) replaced the text "web application" with "web app" in the page menu but the help, keyboard shortcuts and about:applications still have the previous wording. This change updates those to use the same wording as in the page menu.

See also the initiative to replace "application" with "app": Teams/Design/initiatives#123 (closed)

The notes "It is not possible to create/delete Web Apps if you are using Flatpak to run Web" are removed from the help pages as these are possible.