Limit pagination window to 2

* The default pagination window made the number of page links on screen
unruly, and at thinner page widths would break the layout.
* See https://github.com/amatsuda/kaminari
Reword new mapping button to add mapping

* Switch from new to add, but leave 'new' in place on create pages/forms
* Add a plus icon
* Move button to a partial
Add a mappings count header to index

* Show a total count for all mappings
* It helps illustrate how much a list has been filtered
Add bootstrap page header classes

* On sites and mappings indexes use the bootstrap page header class on
the H1
* This increases vertical spacing and adds a light horizontal rule
Remove "no mappings" message

* With a header containing the mappings count, this message in the
filter label is no longer necessary