Add ability to restrict team creation to emails from domain.

Description

1) Create a config setting (to be later moved into database and admin console) to restrict team creation to emails from a specific domain, e.g. @example.com

2) Add an error message if email in team creation screen (attached) doesn't contain a valid domain, but do not reveal the domain--example: "Email must be from a specific domain (e.g. @example.com). Please ask your systems administrator for details."

QA Test Steps

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Mana

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Assignee

Corey Hulen

QA Assignee

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Reporter

Ian Tien

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Fix versions

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Mattermost Team

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Sprint

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Components

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Configure