Change the "Invalidate search index" process to a batched "Rebuild search index"

I like the neatness of the first screenshot that @jenlampton proposed on #1321 (comment).

Placing the same warning message twice on the search settings form:

and then again on the confirm page:

looks really too excessive.

I do understand the initial concern for some people who could press the button without realizing the consequences. However, in fact the consequences are not so scary, because (1) no data is lost anyway and then (2) those who pressed the button will see the warning on the confirm page anyway.

Making the button outstanding with the scary text in red color was sufficient enough, so if possible please remove the description text.