Grant access to Google Analytics based on membership in Slack channel

Related: https://github.com/CivicTechTO/civictechto-scripts/issues/7

Right now, access to Google Analytics is very limited, but we should share more freely, since the insight can be really important. Here's the current access list (just added @candle16):

Operating principles (generally for giving access to services in an automated way):

- Automatically giving people access should be well-considered: sharing is good, but accidents or compromised could really hurt us if people have the higher privileges
- If the privileges are high (full edit access on Google Analytics or whatever), then the Slack channel that this is working from should be private and require an invite. (we currently are doing it this way)