Re-work calendar view

As discussed in issue #223 (which has pictures that better explain
the changes here), this change:
- Removes the date from individual events, instead centering a
  title at the start of each day with the date (which uses a new
  configurable color, `wtf.mods.gcal.colors.day`)
- Consolidates 3 lines per event down to 2, moving timestamp to
  front of each event
- Makes the time-until-event text turn red when under 30 minutes
  (wasn't discussed in the issue but was another thing I added
  locally for this, feel free to discard if unwanted)

New format is:
```
           Monday, Jun 25
 x 13:00 Super Cool Meeting Title 2h
 Event location

 x 14:00 Also Super Cool Meeting 3h
 Event location

          Tuesday, Jun 26
 ...
```

wtf/utils.go

@@ -12,6 +12,8 @@ import (
 
 const SimpleDateFormat = "Jan 2"
 const SimpleTimeFormat = "15:04 MST"
+const MinimumTimeFormat = "15:04"
+const FullDateFormat = "Monday, Jan 2"
 const FriendlyDateFormat = "Mon, Jan 2"
 const FriendlyDateTimeFormat = "Mon, Jan 2, 15:04"
 const TimestampFormat = "2006-01-02T15:04:05-0700"