Improve embedding html and templates

[radiac]

Two related issues:

• I'm finding often want to embed html pages in the file alongside the code. Currently the only way is to define it in a constant at the top, or put it in the return value in the view - both of which obscure/bloat the code.
• I'm finding I often want to render and return template strings, and although the boilerplate for that isn't particularly cumbersome, it is something I usually have to look up.

I think these can be addressed in one solution:

• Defer html/template definition - set them at the bottom of the file by attaching to the app (an object already known in the view)
• Do it in a way we can identify convert should write it into an actual html template file
• View functions can render those templates
• Ideally other templates can also refer to those in extends and include tags

Suggested syntax:

app = Django()
@app.route("/")
def index(request):
    return app.render("index.html", context=...)

app.templates["index.html"] = "{% extends 'base.html' %}{% block content %}Hello {{ target }}{% endblock %}"

Build this using or on top of django.template.loaders.locmem.Loader, and add this as the priority first loader in settings

Here app.render would be a wrapper for django.shortcuts.render, which saves an import and inserts the current request object - I think that should be available in the app. If not, maybe importing django.shortcuts.render would make more sense.

When we run convert, it can then write these templates out into text files, and rewrite app.render into a straight call to render. It will also need to remove our loader from the settings.