Add Refresh ability to RHS

[mattermod]

The GitHub plugin RHS lists notifications, pull requests, assigned issues and review requests. It currently displays the data that is available from the current state of the store but does not have the ability to refresh the data in case is outdated. Add support to Refresh the data by:

1. Calling Refresh whenever the RHS bar is opened to guarantee latest data is available.
2. Add an action icon on the title bar to offer the ability to manually refresh as needed (similar to the one currently available in the team sidebar buttons). Locate the icon in the RHS section title right aligned (in screenshot below, in the “Your Open Pull Requests” title header).

Current RHS:

Sidebar Buttons:

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JIRA: https://mattermost.atlassian.net/browse/MM-18879

[manland]

I work on it!

[hanzei]

Thanks @manland 👍