Draft: Add service desk ticket controller

What does this MR do and why?

Splitting !127482 (merged) into smaller parts

This MR adds the following changes:

• Create a new tickets controller with (for now) show action, to render a new work item type
• Redirects from /service_desk/tickets/:iid to /issues/:iid if "service_desk_ticket" feature flag is disabled

The MR does not move the "index" view of service desk issues to a new controller. We're planning to change that in one of the next MRs.

Screenshots or screen recordings

When the feature flag enabled, we display both service desk tickets: with work item type "ticket", and "legacy" issues created by support bot.

When the feature flag disabled, we display only "legacy" issues.

Tickets#show	feature flag enabled	feature flag disabled

How to set up and validate locally

1. Make sure you have Service Desk configured and enabled. There is a video describing the process https://www.youtube.com/watch?v=SdqBOK43MlI
2. Enable feature flag from the rails console: Feature.enable(:service_desk_ticket)
3. Send an email to an email address specified on Monitor > Service Desk page.
4. Open a created ticket

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.