Allow user to specify single index to reindex when using reindex feature - backend

Problem

There may be a time where you only want to reindex one index. For example changing the number of shards for a single index. At present our reindex button will go ahead and reindex all the indices. This could be wasteful and will mean that we won't be able to use this feature ourselves under such circumstances.

Solution

When performing a reindex the user should specify which indices they want reindexed.