Project Board support

[GlancingMind]

Upstreamed here: git-bug#637

Implement a project board, to have a better overview of which Issues are
currently worked on or which ones still need to be resolved. The board will
have multiple columns, where each columns represents the current state of an
Issue. An Issue will always be only in one columns and may be draged from one
column to another to alter it's state.
E.g.

[TODO]     [WIP]   [Requires Review]   [DONE]
Issue2     Issue1  Issue5              Issue4
Issue3                                 Issue6

The columns of the board may be implemented with labels. Then a column
represents a query for all Issues with the specific label.
E.g. The [WIP] column would list all Issues with the assigned label WIP.
NOTE: As an Issue can only be in one column at a time, it must be ensured that
the Issue never has labels assigned which are also other columns in the board.

[MichaelMure]

Kanban are indeed a common feature of bug tracker. Obviously this will need to be be supported in git-bug's core, possibly with its own data-structure.

As for #11 maybe you could implement the graphQL API with a mock (without persistence) to experiment with that in the webUI. I've noticed https://github.com/nishantpainter/personal-kanban that could fit the bill quite nicely.