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

[mattermod]

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:

/preferences get <name>
/preferences update <name> <value>
/preferences delete <name>

That interacts with the preferences for the current user.

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JIRA: https://mattermost.atlassian.net/browse/MM-25465

[hcwong]

Hi can I take this?

[hanzei]

Sure, thanks would be great @hcwong. Please let me know if you have any questions.

[jfrerich]

This issue appears stale and I’m adding the Up For Grabs label. If work is ongoing, please remove the label. If there is an existing PR, please also link it to the issue. Thanks!