[#3028] Add `followee_list` and `followee_count` action functions
[#3028] Add search filtering to followee_list
[#3028] Change format of dicts returned by followee_list

Match the format requested by @johnmartin for the frontend
[#3028] Refactor a couple of activity streams methods

Refactor _activites_from_users_followed_by_user_query() and
_activites_from_datsets_followed_by_user_query(), make them use an
SQLAlchemy union of _user_activity_query() for all users followed by the
given user and _package_activity_query() for all datasets followed by
the given user, respectively.

This just ensures that what appears in a user's public activity stream,
for example, is exactly what will appear in your dashboard activity
stream if you follow that user. Better than having two different
implementations of the SQL for this query.

_activites_from_groups_followed_by_user_query() already works this way.
[#3028] Fix a mistake in followee_list
[#3028] Document an undocumented param
[#3028] Update a comment
[#3028] Fix a docstring
[#3028] Dashboard now shows dropdown for filtering of activity stream

Still todo:

* Add header for context of specific item active within dropdown
* Snippetize the followee list
* Searching the dropdown (with JS enabled)
[#3028] Snippetized followee dropdown and added context to dashboard when filtering
[#3028] Followee dropdown on dashboard is now searchable via ajax
[#3028] Related context now has better arrow from right hand side
[#3028] My datasets on dashboard now look's nice
[#3028] Small tweak to user dropdown in header
[#3028] Added some doc blocks
[#3028] Updated main.css
[#3028] Small textual tweak
[#3028] Small textual tweak
[#3028] Fix for dropdown button extending too long for long label contexts
[#3028] Add 'num_datasets' to group dicts

Add 'num_datasets' to the dicts returned by group_show().

This is convenient for some javascript that has to show information
about groups.

This fixes an issue where the number of datasets of a followed group was
not appearing on the user dashboard.