Custom intro text for a system, team or channel, similar in set up to Custom Tutorial Text with markdown support and preview, that is displayed to a user when joining a specific system, team or channel, and the user needs to hit "Okay" to dismiss. The user should be able to find the message in the system, team or channel settings to review the info.

Requesting this because often organizations want to share more about the purpose of their Mattermost deployment, and include different links, e.g. link to naming conventions to use for channels, an internal user's guide to the key features in the system intended to be used, perhaps a video introduction from the leader implementing the project, etc.

NOTE: This is proposed as a Team Edition feature should be deployed in a way that is friendly and helpful, not as a way to force team members to accept legal agreements.

Propose the following text be displayed:

Note: This feature is not intended to deliver legal notices. No record of interacting with content here is recorded in the database.  

For legal notices, see: https://mattermost.uservoice.com/forums/306457-general/suggestions/18595561-ee-legal-notice-presentation-for-system-team-and