Add django-admin startapp template

[dalrrard]

Description

django-admin has a --template flag that lets you specify a directory to base new apps off of. I've added a startapp_template directory to the top level directory, updated the post gen hook to remove any files that may not be needed, and updated the documentation for this new process.

Checklist:

• [x] I've made sure that tests/test_cookiecutter_generation.py is updated accordingly (especially if adding or updating a template option)
• [x] I've updated the documentation or confirm that my change doesn't require any updates

Rationale

This lets people semi-automatically create new apps without doing a lot of manual file moving and editing.

Fix #1876 Fix #1725 Fix #1505 Fix #881

[Manjit2003]

Please approve this :(

[browniebroke]

I'm a bit on the edge with this: while it's solving the problem, I find it a bit odd to have an new app template in the repo. After all, the startapp command accepts an URL as template. But maybe having it in the repo means that it can evolve with each person taste...