context_processors: Created basic context processor for setting base template

[Arfs6]

Inspired by the tips of django-htmx docs, I created a context processor that sets the base template to a partial template if the request is a htmx request, else to a full template.
This pull request currently has a basic implementation with some hard-coded values that will not be suitable for django-htmx.
The hard-coded values are of both the base_template variable and it's value. To eliminate this, I am going to make the function to look for both values in the settings.py file. For example:

DJANGO_HTMX_BASE_TEMPLATE = {
    "variable_name": "base_template",
    "values": {
        "full": "_base.html",
        "partial": "_partial.html"
    }
}

Any suggestions for the naming of the variable or the structure of the dictionary is welcomed.

If the variable or some of it's values doesn't exist, I will use the above as the default.

Another thing is to allow a user to override this values, perhaps by calling a function, but that can wait.
Now, I need green light to proceed. What do you think?

[adamchainz]

I’m glad that you’ve managed to follow the tip, but I don’t want to add a context processor to the package. It’s just complicating things to add, test, document, and maintain a 5 line function. Perhaps we could document making a context processor instead.