Declarative url_for construction

[jace]

coaster.sqlalchemy.UrlForMixin has made view construction much simpler by allowing for usage in the form of {{ ob.url_for() }} instead of the longer {{ url_for('ob_view', name=ob.name) }} syntax.

However, the models now require a url_for method which is aware of each view that applies to the model. Since models are lower level than views in our hierarchy (models < forms < workflows < views) and lower levels are by intent not supposed to be aware of higher levels, this has been a bad compromise.

UrlForMixin should provide a is_url_for classmethod that can be used to decorate views, to indicate that this view applies to it. Sample use:

from coaster.views import load_model
from .models import Post  # Instance identified by Post.name

@app.route('/view/<post>')
@Post.is_url_for('view', post='name')
@load_model(Post, {'name': 'post'}, 'post')
def post_view(post):  # This expects to receive the Post object
    pass

Like load_model, is_url_for will need assistance knowing what parts of the post are required to construct the URL, but in reverse, including sub-attributes like self.parent.name. To support this, it accepts keyword arguments after the "action" name, where the value of each argument is a string naming the attribute to extract from self, or a tuple containing a recursive set of attributes (like ('parent', 'name')). As a shortcut, a dotted string may be provided, which is broken down into a tuple at decorator-load time.

[jace]

is_url_for needs to account for two things:

1. @app.route may specify a different endpoint name, in which case we can't use the function's name as the endpoint name.
2. If the view is in a blueprint, we need to call Flask's url_for with <blueprint_name>.<endpoint_name>, but we don't know if this function is in a blueprint. The only distinction is that it has @blueprint.route instead of @app.route.

Given these constraints:

1. is_url_for should accept blueprint and endpoint as optional parameters, OR
2. If app.route or blueprint.route leave a marker on the function, it should discover the blueprint and endpoint name from that (most likely they don't because the same function could be used by multiple routes).

[jace]

Since blueprint routes are referred to as <blueprint>.<endpoint> in calls to url_for, we don't need them as separate parameters. is_url_for can take a single endpoint parameter that should be a dotted value if used on a view inside a blueprint.