Mattermost-server: User autocomplete should match on spaces

[mattermod]

Allow user autocomplete to match on terms with spaces. For example: Full Name

Expected: Similar to how channel autocomplete works. ie Once triggered with @, as long as there is a matching entry the autocomplete stays open, unless dismissed with the ESC key

Observed: User autocomplete is dismissed after a space is entered

Note: this should be done on both the webapp and mobile app autocomplete

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JIRA: https://mattermost.atlassian.net/browse/MM-19761

[allenlai18]

ill take this

[sudheerDev]

@allenlai18 Thanks 🎉