Allow url tags to save result to context variable

[Henrik Vendelbo]

When using translated blocks template tags cannot be embedded, but variables can. This change would allow

{% url about as about_url %}

Which outputs a blank string and saves the url in the context under 'about_url'

[Henrik Vendelbo]

docs,comments,code

[Henrik Vendelbo]

defaulttags.py changes. same as second half of the first file

[Henrik Vendelbo]

dupe r8716