[#781] Begin improving IFacets docs

Better docstrings and an example extension for IFacets. Both still need
some work..
[#1301] Adds skip to content accessibility block
[#1301] Adds header label block and flash message inner
[#1301] Added a couple of template blocks to the activity_streams snippet
[#1301] Added template blocks to group_item.html
[#1301] Adds template blocks to package_list.html
[#1301] Added a few template blocks to the facet lists
[#1301] Added more template blocks to package_item.html snippet
[#1301] Adds blocks for follow button classes
[#1301] Adds a load of template blocks to the package info template
[#1301] Improves the package read page snippets
[#1301] Adds template blocks to tag_list
[#1301] Adds template blocks to resource listings
[#1301] Adds more blocks to the resources snippet
[#1301] Adds template blocks to the resource_read template
[#1301] Adds template blocks to the package search and search form
[#1301] Adds more template blocks to sidebar on the user profile
[#1301] Adds blocks to group and org based templates
[#1301] Adds blocks to the package form buttons
[#1301] Adds hanging enblock