Support milestones

[GlancingMind]

Implement Milestones. A milestone describes a bigger task, which requires
multiple issues to be resolved, possible in a given set of time.
This means, a milestone may have a deadline in which all the Issues
may be completed. The deadline shall currently have no effect on the milestone
and simply provide a hint.

• Add a description to a milestone.
• Add Issues to an existing milestone. This may be done by using special
  "milestone-" labels. The user should then be able to see all current
  milestones within a new menu. The milestones shall have a completion-value,
  which will be a percentage determined by all associated issues and their
  open/close-state. This means a milestone should be 100% complete, if all
  assigned Issues are closed.

Like Issues, milestones may be active or closed as needed.
NOTE: Maybe milestones can be implemented as Issues, with only some fanzy UI
variation (progressbar). Could extend go code to add milestones as composite of the composite-pattern

[MichaelMure]

As you noticed, milestones and labels are conceptually very close. That said, they should probably have their own operation in git-bug's core instead of piggy-backing on the labels. That's a non-trivial amount of code to write, but that would be a lot of copy-pasting from the label's code.

If that's out of scope for your project maybe an intermediary solution would be to add the milestone's specific element in the GraphQL API and then map those into the label system? That way the webUI would stay untouched once someone actually implement that in the core.

[GlancingMind]

Moved to nice to have, as the time could be spent for more significant issues and the implementation is currently unclear.