Support displaying time in absolute format

This adds two config items: useAbsoluteTimeFormat (boolean) and
absoluteTimeFormatMinAge (string, number + unit ('d'|'h'|'m'|'s')).

When `useAbsoluteTimeFormat` is true, the Timeago component will display
absolute time if the time is at least `absoluteTimeFormatMinAge`
from now. If `longFormat` prop is true, the fully formatted time
is displayed. Otherwise, the format is determined by the `time` prop:
(1) if `time` is on the same day of now, display hour and minute;
(2) if `time` is in the same month of now, display day and hour;
(3) if `time` is in the same year of now, display month and day;
(4) otherwise, display year and month.

If it should display relative time, the format is the same as before.

src/modules/instance.js

@@ -119,6 +119,8 @@ const defaultState = {
   closingDrawerMarksAsSeen: true,
   unseenAtTop: false,
   ignoreInactionableSeen: false,
+  useAbsoluteTimeFormat: false,
+  absoluteTimeFormatMinAge: '0d',
 
   // Nasty stuff
   customEmoji: [],