Use 'pageblock' instead of 'block' as django template var

[nikolas]

Because 'block' has the potential to conflict with django's 'block'
templatetag.

Closes github issue #29

[thraxil]

How do you propose dealing with backwards compatibility?

[nikolas]

Thanks for pointing that out. I hadn't thought of that.

I propose that we maintain backwards compatibility by keeping the block variable accessible, with something like this, each time the new pageblock variable is defined:

{% with block=pageblock %}
    ...
{% endwith %}

That way we can slowly migrate our apps from block to pageblock without breaking anything.

If this turns out to cause more problems than it solves we can forget it. I just worry that using block as a variable has the potential to break things.

[sdreher]

Where would the {% with block=pageblock %} definition be placed to maintain compatability?

[nikolas]

In all of the templates that this PR updates. I'll add another commit to reflect that proposal.

[nikolas]

Updating after our conversation to keep anders in the loop: I forgot there's also block getting set in the context as well, which is probably why the tests here are failing (the updated context var here causes that: pagetree/templates/pagetree/testblock.html).

In summary, if we wanted to go in this direction, we would be supporting pageblock as well as block in the context variables. That way it's backwards-compatible. And in case block happens to cause any problems in an app down the road, then we are free to switch it to use the pageblock context var.

[sdreher]

@nikolas - here's where block is being set in the context. https://github.com/ccnmtl/django-pagetree/blob/master/pagetree/models.py#L704