Improve installationStore

[connorads]

https://github.com/connorads/lockbot/blob/master/src/handlers/slack/infra.ts#L24

• Refactor into DynamoDB class?
• Store created/updated timestamp?

[connorads]

Maybe wait for #82 as Bolt 3 might change things with the enterprise/org installation stuff

[connorads]

Didn't end up doing the enterprise stuff in #82
Created a new issue for it #108