Fix DeadlineExceededErrors for leaderboard page

[sjaensch]

We're seeing a couple of DeadlineExceededError: The overall deadline for responding to the HTTP request was exceeded. entries in our error log; full stacktrace below. We'll need to find a way to make the leaderboard calculation more efficient. Relevant Google documentation.

Traceback (most recent call last):
  File "/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/runtime/wsgi.py", line 267, in Handle
	result = handler(dict(self._environ), self._StartResponse)
  File "/base/data/home/apps/s~yelplove/1-7-1.399540497034144168/lib/flask/app.py", line 1836, in __call__
	return self.wsgi_app(environ, start_response)
  File "/base/data/home/apps/s~yelplove/1-7-1.399540497034144168/lib/flask/app.py", line 1817, in wsgi_app
	response = self.full_dispatch_request()
  File "/base/data/home/apps/s~yelplove/1-7-1.399540497034144168/lib/flask/app.py", line 1475, in full_dispatch_request
	rv = self.dispatch_request()
  File "/base/data/home/apps/s~yelplove/1-7-1.399540497034144168/lib/flask/app.py", line 1461, in dispatch_request
	return self.view_functions[rule.endpoint](**req.view_args)
  File "/base/data/home/apps/s~yelplove/1-7-1.399540497034144168/util/decorators.py", line 19, in decorated_view
	return func(*args, **kwargs)
  File "/base/data/home/apps/s~yelplove/1-7-1.399540497034144168/views/web.py", line 136, in leaderboard
	top_lovers, top_lovees = logic.love_count.top_lovers_and_lovees(utc_week_start, dept=department)
  File "/base/data/home/apps/s~yelplove/1-7-1.399540497034144168/logic/love_count.py", line 26, in top_lovers_and_lovees
	employee = employee_key.get()
  File "/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/ext/ndb/key.py", line 572, in get
	return self.get_async(**ctx_options).get_result()
  File "/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/ext/ndb/tasklets.py", line 383, in get_result
	self.check_success()
  File "/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/ext/ndb/tasklets.py", line 378, in check_success
	self.wait()
  File "/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/ext/ndb/tasklets.py", line 362, in wait
	if not ev.run1():
  File "/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/ext/ndb/eventloop.py", line 268, in run1
	delay = self.run0()
  File "/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/ext/ndb/eventloop.py", line 230, in run0
	callback(*args, **kwds)
  File "/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/ext/ndb/tasklets.py", line 414, in _help_tasklet_along
	try:
DeadlineExceededError: The overall deadline for responding to the HTTP request was exceeded.