WIP: Backend for Time Tracking

REF: #985 (closed)

Tasks

• Add estimate slash command

  • Make it available only to project members
  • Parse entered raw duration to minutes
  • Add new column on Issues/MergeRequests to persist the estimate in minutes
  • Overwrite current estimate when a new one is provided
  • Add system note

• Add spend slash command

  • Make it available only to project members
  • Create new database table to handle time tracking entries
  • Add ability to deduct spent time
  • Add system note

• Add remove_estimation slash command

• Add remove_time_spent slash command

• Add specs