Mattermost-server: Implement datetime picker in Interactive Dialogs

[mattermod]

Currently there is no way to pick a date or datetime in interactive dialogs. The current elements can be found here: https://docs.mattermost.com/developer/interactive-dialogs.html#elements

Issue created from a message in Mattermost.

Note that this will need to be implemented in the webapp and the mobile app

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JIRA: https://mattermost.atlassian.net/browse/MM-28456

[papakaliati]

Mine for the taking.

[mickmister]

FYI this will need to be implemented in the webapp and the mobile app. Thanks for taking on the task @papakaliati!