Add support for slack enterprise id

[elisherer]

Inside activity and for fetching token and bot user by it, addresses #1962

By @reesesm2000

If a bot is installed in an Enterprise Grid, it can be accessed from any of the teams in the grid. When the bot is installed, it only gets to know about the first team where the install is done therefore causing a team not found error when accessed from a separate team in the grid. Making the changes below allow for automatically adding the team id if it belongs to an already registered grid not needed the app to be reinstalled.

I followed the changes he suggested, but I changed the arguments of getTokenForTeam and getBotUserByTeam to include the entire activity (in addition) instead of just the additional enterprise id to support custom future token logic.

[ghost]

All CLA requirements met.

[stale[bot]]

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