Adds a new 'Gradual reindex' link to the search areas page for each
area. When clicked, this takes you to a confirm prompt, and then
adds each context from that search area to the indexing queue.

The search areas page now displays the 'Additional indexing queue'
(if it is non-empty). The table shows the first 10 items in the
queue, and it also indicates the total number in case there are
more. (I don't think people really need to see the entire
contents of it, so I didn't implement paging.)