Remove insert_column workaround

LAVA-1601 - django-tables2 has migrated to OrderedDict
from the deprecated SortedDict. This means that filters
need to create columns in the __init__ in the order in
which the columns are to be displayed, rather than re-ordering
later.

To manipulate the base columns safely, all columns in such
tables need to be instance variables, not class attributes.

Change-Id: I9921228571733e0605c52d79740903adad3d0d3d
(cherry picked from commit cf8514ad6542a6095ec6915c2610b2e4a617fe82)
Fix a regression from reducing the test names in case of one filter charts.

Fixes bug 747 (https://bugs.linaro.org/show_bug.cgi?id=747).

Change-Id: I0fdd1ad7ecc5bd651a048e278f6c32a8137955e7
(cherry picked from commit a632b538a390eae5a41f378f0dc4b6445d3cf449)
Replace mimetype by content_type

It has been deprecated in Django 1.5 and removed in Django 1.7

Change-Id: Iedc0048ba070dc9e7c89f4bb6adf4e51548e3dae
(cherry picked from commit 3d93137d2dcfa2b634c67798f0db2802e99ae8cc)
Return an error when the form is invalid

When an invalid POST request is made, the server was crashing with the folowing
error:

"""
ValueError: The view
dashboard_app.views.image_reports.views.image_chart_settings_update didn't
return an HttpResponse object. It returned None instead.
"""

Return HTTP 400 (bad Request) instead.

Change-Id: If33b65a39f7badfc57a56230dd9d7b6bcbb86a2c
(cherry picked from commit 6be471b3d4a3ff595d0815e2e56caf36d965bbf7)