Add centralised requirement checker

[ibboard]

For improved plugability, requirements should be checked through a centralised "requirement handler". This would allow better handling of Warhammer 40K's roster-style or Warhammer's numbers based on army size restrictions through plugins by giving a central point to add the requirements to. It would also separate out "one blob, one job" functionality by removing functions like CanAddToArmy from classes like UnitType.

[ibboard]

Remove owner so that the team can more easily see bugs. Using default owners seems good at first but is bad practice when a team can be working on any part of the project.

[ibboard]

Move some tickets to v0.2 milestone

[ibboard]

Move tickets to new "confirmed" status to show that they've been checked

[ibboard]

Separate API milestone from UI milestone

[ibboard]

In [898]:

Re #33: Centralised requirement handling

• Use central requirement handler to check each selection

[ibboard]

In [926]:

Re #33: Add centralised requirement checker

• Add test for getting "allows adding" failure messages from handler

[ibboard]

In [927]:

Fixes #33: Add centralised requirement checker

• Implement validation with messages for adding

(Other features added under #27 and not separately marked)