Fix undefined name errors

[jsjiang]

Flake8 reported F821 undefined name errors in following scripts:

ezid/ezidapp/stats.py:62:36: F821 undefined name 'lastM'
ezid/templates/doc/ezid.py:116:1: F821 undefined name 'django'
ezid/templates/doc/ezid.py:169:41: F821 undefined name 'django'
ezid/templates/doc/ezid.py:239:1: F821 undefined name 'django'
ezid/templates/doc/ezid.py:251:28: F821 undefined name 'django'
ezid/tools/dashboard.py:138:40: F821 undefined name 'lastMonth'

The ezid/templates/doc/ezid.py script is not used and can be ignored. The errors on the 'lastM' and 'lastMonth' are from two very similar next month calculation routines.

Define the 'lastM' and 'lastMonth' variables to 'None' before these routines should fix the issue.

    for m in months:
        if m != months[0]:
            # noinspection PyUnboundLocalVariable,PyUnboundLocalVariable
            nextM = incrementMonth(lastM)
            while nextM != m:
                table.append((nextM, {}))
                nextM = incrementMonth(nextM)
        table.append((m, counts[m]))
        lastM = m

    for month in months:
        if month != months[0]:
            # noinspection PyUnboundLocalVariable
            nextMonth = incrementMonth(lastMonth)
            while nextMonth not in months:
                counters[nextMonth] = Counter()
                nextMonth = incrementMonth(nextMonth)
        lastMonth = month