Make an example of using the Custom Status plugin API methods

[mickmister]

With the additions from mattermost/mattermost#17435 (comment), plugins can now access the APIs around managing a user's custom status. This makes it so plugins can:

• Get a user's custom status
• Change the user's custom status
• Remove the user's custom status

The task here is to create slash commands to make use of these newly added methods:

• /demo custom_status get
• /demo custom_status update (...value)
• /demo custom_status delete

Each command should reflect the current user's custom status state.

You will need to deploy the commit from the PR mattermost/mattermost#17435 to have access to the plugin API methods. To checkout this code:

• cd mattermost-server - wherever you have this on your filesystem
• with hub installed, run hub pr checkout 17435