Add category to /spend quick action

What does this MR do and why?

A while back we added the concept of timelog categories.

This MR adds the ability to set timelog category using the /spend quick action.

MR acceptance checklist

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Screenshots or screen recordings

Before	After

How to set up and validate locally

1. From the rails console:
   1. Enable the timelog_categories feature flag.
   2. Create a timelog category: TimeTracking::TimelogCategory.create!({name: 'Support', namespace: Namespace.find(33)})
2. Find or create an issue within that group and log some time with the quick action:

   /spend 1h [timecategory:Support]

3. Check the timelog from the rails console: Timelog.last (you should see the timelog_category_id set).