Demo (Get|Update|Delete)PreferencesForUser in demo plugin

Description

Build on to showcase the use of the new (Get|Update|Delete)PreferencesForUser API. Introduce a new slash command in the demo plugin to perform the following:

That interacts with the preferences for the current user.

QA Test Steps

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Mana

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Assignee

Unassigned

QA Assignee

None

Reporter

Jesse Hallam

Epic Link

None

Fix versions

Mattermost Team

Toolkit

Sprint

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Labels

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QA Testing Areas

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Components

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Configure