[Custom Fields] Navigate to Settings > Work Items > Custom Fields (and foundational BE/DB work for configuring custom fields)

Context

• For context about this problem, proposal, implementation roadmap, or UX Research, see &235
• For context about decisions made during the implementation spike, see #472640 (closed)

UX

[UX] Custom fields design - MVC1 (#461471 - closed)

Acceptance Criteria

• All tasks on this issue