Migrate to Django 1.10

Created by: SunilMohanAdapa

This patch is based on @harry-7 's fix for #544 (closed) which is #546 .

I went through the complete Django 1.10 release notes for deprecations and removals. I fixed all that noticed. Also, since testing also has Django 1.10, this patch proposes depending on it as minimum version.

Created by: SunilMohanAdapa

@harry-7 as continuation of our discussion, see the changes that were required for stronghold application to fix the recursive redirections on login page. Consider doing an additional review of this patch to help @jvalleroy .

@jvalleroy , since we need to fix a current live issue, we have two options:

• Make a release after merging this leading to Plinth 0.10.
• Branch 0.9 and apply first four patches in this series (other two can be ignored). Then make a release from there.

Created by: SunilMohanAdapa

@harry-7 , I have created a bug report for the Django documentation issue we have found.

Created by: harry-7

@SunilMohanAdapa I checked the files and ran through the code that is changed. The patch looks fine for me. I wonder we should update our INSTALL file mentioning that we need django 1.10 now. what do you say ?

Merged. I also updated the Django version requirement in INSTALL and requirements.txt.

Thanks @harry-7 and @SunilMohanAdapa for the quick fixes!