Converts substitutionTimer string to MM:SS

The string used to just list the seconds. It now displays it as MM:SS.
Replaces hardcoded string with a default.

The substitution_interval_timer should be set to something by default
even if it's going to be set through user configuration when the program
starts anyway.
Plays system default alarm when the subsitutionTimer finishes.

Be careful here though, there is no way to make it stop besides closing
the program.