Support full page create flow for work items

Summary

With work items the primary create flow was defined as utilizing a modal, which can allow for the most consistent experience across different contexts including those where leaving the current context is undesirable.

However there are a number of cases where having a full page view is more beneficial:

• Supporting direct links to the create form as /new
• Supporting users wanting a more focused view without space constraints
• Supporting users wanting to shift their context entirely to the new item, including after creation

Proposal

Current iteration (Epics migration)

1. Link the "New epic" button in both the global create menu and the epic page to /epics/new , allowing users to open in a new window or tab (on click, continue using modal).
2. In the creation modal, add a "Open in full page" link pointing to the full page 2. Pass any entered data from the modal into the full page
3. Utilize the full page work item creation view (currently /work_items/new and requiring feature flags), defaulted to epics and with the type selection not shown.
4. On successful item creation, open the new epic in a full page view.

Next iteration (Issues migration)

1. Link the "New issue" button to /issues/new
2. Utilize the full page work item creation view (currently /work_items/new and requiring feature flags), defaulted to issue and with the type selection not shown.
3. On successful item creation, open the new issue in a full page view.

Later iteration (Post-migration)

1. Link each type in the "New" dropdown to the /work_items/new route
2. Utilize the full page work item creation view, defaulted to the selected item type, but retaining the type selector.
3. On successful item creation, open the new issue in a full page view.