Allow usage of severity quickaction on create

What does this MR do?

This makes some changes to allow the /severity quick-action to be used when creating an Incident via the new Incident UI.

Previously this behaviour would 500 due to the code ending up in a loop.

The downside of this change, is that we cannot determine the type of issue before the form has been submitted. That means to show the autocomplete logic of the /severity quick-action we need to enable it for all issues. Upon creation of the issue however, no severity is saved. Since we do not show severity for non-incident issues I don't think this is a problem.

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

Before	After

1. Create an incident via Monitor -> Incidents -> New Incident, or via the new Issues page.
2. On the creation form, use the /severity quick-action i.e /severity 3.
3. The incident will be created and the chosen severity will be set.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #332594 (closed)