Enable Reindexing from Admin Panel

Originally recorded in [MAGETWO-87804](https://jira.corp.magento.com/browse/MAGETWO-87804)

Requires Merchant Documentation: **Yes**

Proposed Release Note:
>This feature allows merchants to reindex specific indexes from the Admin Panel. Before this feature, reindexing was only possible from the command line.
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:bulb: Initial feedback for story provided by Solution Partner [Something Digital](http://www.somethingdigital.com)

### Card:
As a **merchant** I want to reindex one or more indexes from the Admin Panel **so that** I can update my site content without having to use the command line or contact a developer.

### Background:
In Magento 1, it was possible to reindex one or more indexes from the Admin Panel. This feature was deprecated from the Admin Panel in Magento 2, and it's only possible to reindex sites from the command line using the following commands:

Command | Description
------------ | -------------
`indexer:info` | Shows allowed Indexers
`indexer:reindex` | Reindexes Data
`indexer:reset` | Resets indexer status to invalid
`indexer:set-mode` | Sets index mode type
`indexer:show-mode` | Shows Index Mode
`indexer:status` | Shows status of Indexer

Many merchants cannot reindex from the command line, for several possible reasons:
* They do not understand how to use the command line and command syntax
* They prefer to manage their site from a graphical user interface
* They do not have access to use the command line (may be blocked by developer or solution partner)

Merchants who have to contact a developer or Solution Partner to reindex the site may incur additional development cost.

### Feature Description:
In the Admin Panel merchants can view Indexes and status through the `System > Index Management` menu. This page shows all indexes, along with the description, mode (save/schedule), status, and last updated timestamp for each index. Merchant users can select one, more than one, or all indexes using the selection grid.

A new action should be added named 'Reindex' to the existing action drop-down menu. The existing options are 'Update on Save' and 'Update by Schedule'. If the user selects 'Reindex' a button appears labeled 'Submit'. Next to the button is a status that shows the number of records shown and selected in the grid: 'x records found (*x* selected)'. When the user clicks 'Submit' the selected indexes are reindexed.

### Acceptance Criteria:
* User can select one, more than one, or all records from grid
* After user clicks 'Submit' selected indexes are reindexed
* Reindex action is identical to reindex initiated from the command line
* Page is refreshed after 'Submit' click action to show current status of each index
* Success or failure message is shown to user after clicking 'Submit'
* Status of each index is displayed in the 'Status' column (existing behavior)
* Merchant documentation for feature created
* Developer documentation for CLI index commands updated to reference new feature in admin panel

### Additional Information:
Screenshot showing how feature was possible from M1 Admin Panel:


Screenshot from Magento 2 Admin Panel:
* Shows existing Index Management Grid
* Shows in-process state of Product Price Index (PROCESSING)
* Shows summary of user-selected grid items: 10 records found (2 selected)

Start by reviewing the existing System > Index Management page and the listed CLI index commands. Done means merchants can select indexes and reindex them from the Admin Panel, see refreshed statuses and success or failure feedback, and the requested merchant and developer documentation updates are complete.

Written by the indexing model from the issue text.