Work around bad timezone handling in ago library

The ago library doesn't handle datetimes correctly if they have any
timezone except the system's localtime. This can be worked around by
calculating a timedelta and passing that instead of a datetime. This
commit updates the descriptive_timedelta() function to do that.

I've also registered an issue on the library, but I don't know if it's
maintained any more:
https://bitbucket.org/russellballestrini/ago/issues/3

tildes/tildes/lib/datetime.py

@@ -97,7 +97,11 @@ def descriptive_timedelta(target: datetime, abbreviate: bool = False) -> str:
 
     A time of less than a second returns "a moment ago".
     """
-    seconds_ago = (utc_now() - target).total_seconds()
+    # the ago library doesn't deal with timezones properly, so we need to calculate the
+    # timedelta ourselves and only ever call human() using a timedelta
+    delta = utc_now() - target
+
+    seconds_ago = delta.total_seconds()
     if seconds_ago < 1:
         return "a moment ago"
 
@@ -107,7 +111,7 @@ def descriptive_timedelta(target: datetime, abbreviate: bool = False) -> str:
         precision = 1
     else:
         # try a precision=2 version, and check the units it ends up with
-        result = human(target, precision=2)
+        result = human(delta, precision=2)
 
         units = ("year", "day", "hour", "minute", "second")
         unit_indices = [i for (i, unit) in enumerate(units) if unit in result]
@@ -119,7 +123,7 @@ def descriptive_timedelta(target: datetime, abbreviate: bool = False) -> str:
             # otherwise, drop back down to precision=1
             precision = 1
 
-    result = human(target, precision, abbreviate=abbreviate)
+    result = human(delta, precision, abbreviate=abbreviate)
 
     # remove commas if abbreviating ("3d 2h ago", not "3d, 2h ago")
     if abbreviate: