ChatOps calls should record user activity

What does this MR do and why?

ChatOps calls should record user activity as per #413368 (closed)

This MR triggers the user activity service at the same time as updating the usage timestamp on the ChatName record. There were some suggestions¹ about where to place this call, but I selected the FindUserService for the following reasons:

1. It makes sense to me to mark activity on the ChatName and the user in the same place.
2. Rubocop was nudging me away from calling a service from a model, and the service -> service call satisfies the code re-use guidelines².

Changelog: fixed

How to set up and validate locally

1. Create a chat name for a user that has no recent activity in GDK's Rails console:

   user = User.first
   user.chat_names.create(chat_id: 1, team_id: 1)

2. Set up a Slack command integration in one of your projects and copy the hook URL.

3. In your terminal:

   curl --request POST --header "PRIVATE-TOKEN: <your-pat-here>" \
     --url "http://localhost:3000/api/v4/projects/7/integrations/slack_slash_commands/trigger?token=<your-token>&team_id=1&user_id=1"

4. Verify that User#last_activity_on has been updated.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

1. #413368 (closed) ↩

2. https://docs.gitlab.com/ee/development/reusing_abstractions.html ↩