Added a tab 'Logins' to site statistics page that contains
how many total logins for an institution and also how many unique
users have logged in during a certain time period - defaults to
previous calendar month

One can get different results by adjusting the url like so
admin/statistics.php?type=logins&start=2014-01-01&end=2015-01-01
if needing to get a different time period

On upgrade it populates the usr_login_data table with the current
lastlogin time for non-deleted users

Currently usr_login_data only records user id and ctime (for login
time) but one could also add more columns tothe table if needing to
record something that happens once per successful login.

Change-Id: If59b207356894eaced7b9977b80d539a28cb7e56
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>