Add '/dt' slash command

[fgiannar]

PR Description

This PR introduces an additional slash command that will automatically parse a message to convert the time to the viewing member's local time.
This removes the need for an extra question to list channel members in private channels improving the UX in private channel and direct messages.
Furthermore, it improves the UX in cases where we simply want to display a date to everyone viewing that text in their current local time without mentioning all other team members' timezones.

Added functionality

• Adds a new slash command, /dt
• Adds a new endpoint, slash/receive-dt that receives the POST payload from the dt slash command
• Makes use of Slack's Date Formatting in order to display a date in the local timezone of the person seeing the text.

Sample