Correct Canonicals

[hatdropper1977]

The old version of Attila Hard Codes the Canonical values for the Categories, Authors and Tags pages.

This PR uses the CATEGORIES_SAVE_AS, AUTHORS_SAVE_AS and TAGS_SAVE_AS values set in pelicanconf.py.

For example:

{% block canonical_url %}<link href="{{ SITEURL }}/authors" rel="canonical" />{% endblock canonical_url %}

Becomes:

{% block canonical_url %}<link href="{{ SITEURL }}/{{ AUTHORS_SAVE_AS }}" rel="canonical" />{% endblock canonical_url %}

[arulrajnet]

Thanks @hatdropper1977