Implement 'Create/Edit Search Index' functionality

[dmitrizagidulin]

Implement the ability to create new Search Indexes, or edit/delete existing ones, using the explorer cluster proxy URLs and the existing Riak REST API -- see http://docs.basho.com/riak/latest/dev/references/http/store-search-index/

Note: Edits to an index (such as changing the search schema it uses) does not take effect automatically, the user still has to reload the index, see issue basho-labs/riak_explorer#88 and #22.

Requires basho-labs/riak_explorer#89 to be really useful.