Add notion of activities in time tracking

Description

Redmine has a concept of activities that lets you add time tracking on an issue for a specific activity in this issue (designing, testing, documenting, and other non-strictly programming activities).

Proposal

Add @user and message parameters to optionally spend time on behalf of someone else/add a related message

User adds time spent, the optional user, and the optional message

The system note shows up with the message below it and indented (reusing styles from discussion comments)

Design notes

• The indented message should reuse-styles from Discussion type notes
• If a user clicks Start discussion when inputting the quick action, everything should act just the same as if the user clicked Comment (exactly how the other quick actions work)

Reference:

• comment on hacker news
• Redmine on the topic