Rename keys.IssueAssign to keys.Assign (used for both issues and PRs)

[joshuacrass]

Problem:
keys.IssueAssign in keys.go is bound to a and is used in both the issue detail view and the PR detail view (model_prs.go:768). The name IssueAssign is semantically wrong for the PR case. If the key is ever refactored to behave differently per context, the PR case would be silently broken because it references a key named for a different resource type.

Solution:
Rename IssueAssign to Assign in keys.go and update all references in model_issues.go and model_prs.go.

Acceptance Criteria:

• keys.IssueAssign is renamed to keys.Assign
• All references in model_issues.go and model_prs.go are updated
• go vet ./... and go test ./... pass