Update Index Management to support hidden indices

[cjcenizal]

Context

Per convo with @gwbrown and @jaymode, "system indices" will be a formal concept introduced into the stack in 8.0. For the most part, these system indices are for specialized consumption by particular actors in our stack and not intended for the end user.

As part of the introduction of this new concept, in 7.7 ES has added another concept of "hidden indices". These are indices that have been created with the hidden: true setting like this:

PUT hidden
{
    "settings" : {
        "hidden": true
    }
}

Currently, Index Management treats all indices that start with a dot as a "system" index. This will change in the future. Some of these indices will become true system indices, which won't be surfaced in Index Management at all. Some of them will become hidden indices.

Proposed changes

Let's make the following changes in 7.x:

1. Retrieve all indices, including hidden indices. Notice that the logic that currently does this (fetch_indices.ts) uses the cat API, but we'll need to change this to use the index API: GET */?expand_wildcards=hidden,all.
2. Change the toggle label to "Include hidden indices".
3. Update our filter logic to treat indices that begin with a period as hidden indices, even if they don't have the hidden setting.

The end result will be that indices which are intended to be hidden (those that begin with periods) will be presented as if they are hidden, even if they haven't been migrated to use the hidden setting yet.

In 8.0, we can alter the logic in step 3 to only filter indices with the hidden setting. By this major, all of our internal indices will have been migrated to be either true system or hidden indices, and those that begin with periods will be user-created.

[elasticmachine]

Pinging @elastic/es-ui (Team:Elasticsearch UI)