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

Status

Mana

None

Assignee

Maria Nuñez

Reporter

Jason Blais

Fix versions

Mattermost Team

Toolkit

Sprint

None

QA Assignee

Prapti Shrestha

QA Testing Areas

Plugins