[MM-57197] Weekly active users

[JulienTant]

Summary

Add weekly active users metrics, letting us know how many users sent/received DM/GMs (aka chat in the MSTeams language) through the plugin in the last 7 days.

• For each message sent through the plugin, we update the sender user line to set a new LastChatSendAt.
• For each message received through the plugin, we update each person who will receive the message, excluding the original sender.
• Created the 2 associated gauges.

Maybe some changes deserved to be in their own PRs but I wanted to "fix" some things I've seen:

• store.GetStats() has been removed in favor of having a list of individual queries
• api.siteStats() now uses one of those new methods from above.

Ticket Link

https://mattermost.atlassian.net/browse/MM-57197

[JulienTant]

@lindy65 I marked this PR as deferred because I feel like metrics are easier to debug with grafana!

[JulienTant]

Hey @sbishel when you have a minute can you take a look? 👀