Provide more information before starting a reindex

When a JIRA Administrator starts a reindex (e.g. after modifying a custom field searcher) there's no guidance about how long this might make JIRA unavailable to all the other users. A useful piece of information to display would be how long the last complete reindex took, as an estimated duration for the next reindex. This information would reassure administrators that it's ok (or not) to start a reindex.