Some search index bots have a tendency to get stuck on datagrid pages.

They navigate through pages, click sort/unsort and column edit links,

and end up generating mass amounts of URLs to index.
This change works to control this through sane defaults and full opt-out

of search indexing.
The sort and column choice links now use role="button" and

rel="nofollow noindex". The role="button" should prevent indexing

by itself, but we use both to cover bases.
Pagination buttons now use the same nofollow noindex for the "Last"

link, to prevent jumping to the end of the list where queries are most

expensive.
Other pagination buttons set rel to first for the first page, last

for the last page, next for the next page,and prev for the previous

page. Not all engines care about this (Google ignores them), but some

do, and can help search engines with their indexing choices.
Datagrid pages define a canonical URL that excludes columns, sort,

and others, helping to de-index previously-indexed pages with these

options set, and reducing the indexing queue.
Standard query string arguments for sorting/unsorting now use sort order

instead of dictionary order, keeping URLs stable.
Search indexing can also be disabled entirely for a datagrid by setting

allow_search_indexing = False. In this case, all pagination links will

use rel="nofollow noindex", and the datagid pages will set the same in

a <meta> tag. This is particularly useful when there are multiple

datagrids that ultimately cover subsets of a larger list of items.
In the process, a small optimization was made to the code determining if

there should be a First Page or Last Page link shown. We were searching

the entire list of pages for page 1 or page <length>, which was

unnecessary. We now just check the first and last page numbers in the

display list, respectively.

Unit tests pass.
Tested this with Review Board's datagrids, checking the resulting

HTML with search indexing on and off.