Context modifiers: Search for context_modifiers.py in registered Django apps

[ababic]

As reported by @thibaudcolas on #141 - context modifiers registered in pattern_contexts.py (as indicated in docs) are not being picked up, because nothing is currently loading those modules.

I quite like the idea of using pattern_contexts.py as a pattern in apps, so these changes trigger a search for such modules when get_for_template() is first used (as late as possible, to avoid 'app not yet loaded' issues)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have added an appropriate CHANGELOG entry

[thibaudcolas]

Thank yoouuuuuuuuuuuuuuuuuuuuu