Strings containing time intervals should be pluralized

[Dylan McCall]

Right now when we have a time interval inside another string, we do something like this:

string delay_string = NaturalTime.instance.get_simplest_label_for_seconds (time_since_start);
string label = _("You were due to take a micro break %s ago").printf (delay_string);

Instead, we need to use ngettext so translators can provide a plural form of the wrapping string.

Problematic strings:

helper/microbreak/MicroBreakView.vala
"You were due to take a micro break %s ago"

helper/restbreak/RestBreakView.vala
"%s remaining in your break"
"You were due to take a break %s ago"

settings/MicroBreakType.vala
"Take a break from typing and look away from the screen for %s. …"

settings/RestBreakType.vala
"… Your break has %s remaining. I’ll remind you when it’s over."