Add support for plugins to override right-hand sidebar

Description

Few use cases:

1) (from customer) Click on an IP address in a Mattermost channel: this address opens the right-hand side panel with more information pulled from an internal database like the OS, browser, etc. Useful for troubleshooting.
2) Clicking on a Google map link or image preview opens the right-hand side panel with more information on the location
3) (from community) Displays a list of GIFs a user can choose from after invoking a giphy slash command

QA Test Steps

You can test the changes with the demo plugin, see https://github.com/mattermost/mattermost-plugin-demo/pull/31 for details

Reporter

None

Mana

None

Assignee

Maria Nuñez

QA Assignee

Prapti Shrestha

Reporter

Jason Blais

Fix versions

Mattermost Team

Toolkit

Sprint

None

Labels

None

QA Testing Areas

Plugins

Components

None
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