Tidy up implementation of custom assertions

[mjtamlyn]

The custom assertions provided by Django have a significant amount of duplication between the true and false cases - e.g. between assertContains and assertNotContains. There appear to be subtle difference and it concerns me that perhaps some of these are bugfixes which have only been applied to one of the methods.

For example, assertNotContains seems to not handle streaming responses.

[ersran9]

I've written an initial patch ​https://github.com/django/django/pull/1507 . I'm not really sure if this is the way to go forward. If it is, then I'll move forward with other functions that has the same problem.

[Tim Graham <timograham@…>]

In a2374bcf476b49441793f1ad213126b07543da54:

Fixed #20918 -- Tidied up implementation of custom assertions

1. Moved common logic between assertContains and assertNotContains into a separate function _assert_contains()

2. Moved common logic between assertTemplateUsed and assertTemplateNotUsed into a separate function _assert_templateused()

Thanks Marc Tamlyn for the report and review.