Add template chunk tag, to support apps plugging template chunks.

 - Adds a new template tag: {% app_chunk 'chunk_name' **kwargs %}
   This custom tag is useful for plugging apps using django_mailman3
   together. The tag scans INSTALLED_APPS, inorder, for each app
   except the current one, it looks up whether the
   "<app_name>/chunks/<chunk_name>.html" template exists. If it does,
   it is collected and the result of rendering all the templates
   inorder, with the current context updated with tag keyword arguments
   , if any, is returned.
5 jobs from template-chunks in 2 minutes and 33 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Test
passed #29986893
django-1.10

00:02:01

passed #29986887
django-1.8

00:02:32

passed #29986892
django-1.9

00:01:46

failed #29986903
allowed to fail
django-latest

00:01:28

passed #29986901
lint

00:01:26

 
Name Stage Failure
failed
django-latest Test There is an unknown failure, please try again
    AttributeError: 'module' object has no attribute 'lru_cache'

----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-vd74eB-build/

ERROR: could not install deps [mock, git+https://gitlab.com/mailman/mailmanclient.git, https://github.com/django/django/archive/master.tar.gz]; v = InvocationError('/builds/J08nY/django-mailman3/.tox/py27-django-latest-head/bin/pip install mock git+https://gitlab.com/mailman/mailmanclient.git https://github.com/django/django/archive/master.tar.gz (see /builds/J08nY/django-mailman3/.tox/py27-django-latest-head/log/py27-django-latest-head-1.log)', 1)
___________________________________ summary ____________________________________
ERROR: py27-django-latest-head: could not install deps [mock, git+https://gitlab.com/mailman/mailmanclient.git, https://github.com/django/django/archive/master.tar.gz]; v = InvocationError('/builds/J08nY/django-mailman3/.tox/py27-django-latest-head/bin/pip install mock git+https://gitlab.com/mailman/mailmanclient.git https://github.com/django/django/archive/master.tar.gz (see /builds/J08nY/django-mailman3/.tox/py27-django-latest-head/log/py27-django-latest-head-1.log)', 1)
ERROR: Job failed: exit code 1