Add timer start/stop to task card

Problem/Motivation

As a user I would like a way to record my worklog by starting and stopping a timer via clicking on a task card's timer icon. The timer would be user specific so that multiple people could log hours on the same task

Steps to reproduce

Proposed resolution

This would need several items
An API endpoint to start and stop a user's timer for a card
A timer storage to keep state for one timer per user. A user may have only one timer running at a time. starting a timer on one card will inform the user that another card's timer was stopped as a result of starting the new timer.
An event subscriber that fires when a user stops a timer that assigns the accumulated time to the work log.
Nice to have: a nightly cron that sends an email notification to any user that has an active timer running that has accumulated more than 12 hours (setting?). The email would suggest that they may have inadvertently left a timer running, and provides a link to shut it off and edit the worklog that just recorded.

Remaining tasks

User interface changes

API changes

Data model changes