Implement 'Edit Cached List' button for Buckets and Keys

[dmitrizagidulin]

Once basho-labs/riak_explorer#117 is sorted out, implement the ability for users to edit the cached bucket lists and cached key lists on the api-side.

1. Implement an 'Edit Cached Key List' button on the Bucket view (displayed both in Dev mode and Prod mode).

2. When clicked, this will lead to an edit view with a big textbox, that displays the current cached list (retrieved in plain text mode). When a user hits submit, the client will send a DELETE call to the existing cached list, and then submit a PUT request to the cached list (since currently the PUT appends to the existing list, the DELETE beforehand is necessary).

3. Implement an 'Edit Cached Buckst List' button on the Bucket Type view, with the same functionality.

4. For cases where the cached lists are too large to usefully cut and paste into a browser text box, also display a curl command-line (above or below the textbox) that a user could cut & paste and upload a large file directly via curl. Something like:

   curl -XPUT localhost:9000/explore/clusters/production1/bucket_types/default/buckets/users/keys \
   -H 'Content-Type: plain/text' --data-binary @keylist.txt