Improved refresh of RHS

Nobody has claimed this yet.

The [GitHub plugin](https://github.com/mattermost/mattermost-plugin-github) RHS lists notifications, pull requests, assigned issues and review requests. It currently refreshes on demand, or whenever the GitHub plugin happens to receive an update that forces the client to fetch the latest results.

At this time, not every update is proxied to the GitHub plugin and thus some items that might be expected to disappear instantly – e.g. read notifications, or finishes reviews – stay in the interface longer than might be expected.

Let's introduce improved refresh semantics:

* keep track of when the browser gains/loses focus, perhaps by extending the plugin API to do this, or if we already track this in Redux, surfacing a selector to expose this to the plugin
* if the RHS is open, and the browser regains focus, trigger an automatic update on the assumption the user came back to the Mattermost application and wants new results.
* ~~some items may disappear when the update occurs, but this might be jarring in the user interface: instead, keep these in memory within the component, and render any item not found in the canonical dataset in a dimmed/greyed out fashion~~ (This is getting worked on in https://github.com/mattermost/mattermost-plugin-github/pull/138)

For notifications specifically let's also automatically dim/grey out items when clicked, since it's expected that these items will be marked as unread when the link is clicked, and the user might just be opening in new tabs without leaving Mattermost.

The net results of this might look like:


and it will now be easier for the enduser to click through their TODO list and know exactly where to pick up when they come back.

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JIRA: https://mattermost.atlassian.net/browse/MM-18888

Start by tracing the GitHub plugin RHS refresh flow and how browser focus is represented, including the plugin API or Redux selector options described in the issue. Review PR #138 for the dimmed-item work, then verify that regaining focus refreshes an open RHS and clicking notifications dims them.

Written by the indexing model from the issue text.