Improve spec coverage for boards vuex action [RUN AS-IF-FOSS]

What does this MR do?

• Combine addListNewIssue and createNewIssue action

Why? addListNewIssue vuex action previously lacked any spec coverage and its structure made the action difficult to test (it had a dispatcher that asynchronously returned a promise !58558 (comment 545281209))

addListNewIssue action is responsible for this feature: Screen_Recording_2021-04-05_at_6.30.27_PM

• Remove redundant mutations (!58558 (comment 545326395))

• Make sure to clean up a temporary placeholder issue in case of a request error.

• Use a unique id for the placeholder issue.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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