Template tags rationalisation

This pull request does the following:
 - Merges "pageurl" and "rich_text" tag libraries to create "wagtailcore_tags"
 - Renames "embed_filters" tag library to "wagtailembeds_tags"
 - Renames "image_tags" tag library to "wagtailimages_tags"
 - Deprecates "embedly" tag which has been replaced by "embed"