closes #685 by strictly defining points in time

- today is from 00:00.00 this morning until the end of time
- yesterday is from yesterday 00:00.00 until today
- the other ones also mean pretty much exactly what you would expect
them to mean.
- this week and this month both can be empty because they are already
covered by today, yesterday etc. This is by design.

res/values/strings.xml

@@ -36,6 +36,8 @@
     <string name="yesterday">Yesterday</string>
     <string name="this_week">This week</string>
     <string name="this_month">This month</string>
+    <string name="last_week">Last week</string>
+    <string name="last_month">Last month</string>
     <!-- Translators: show beside messages that are "N minutes old". prefer a short string, prefer abbreviations if they're well-known -->
     <plurals name="n_minutes">
         <item quantity="one">%d min</item>