Split and rename project issue sidebar specs

What does this MR do?

This MR splits some parts of sidebar_spec.rb into separate files in order to diagnose the cause of its flakiness in CI.

(some parts of sidebar_spec.rb have been already split off in !56598 (merged))

More context is in this issue: #323325 (comment 530857949).

file	what changed
sidebar_assignee_spec.rb sidebar_labels_spec.rb	- changed the title of the spec to be more descriptive - replaced let! with let_it_be where appropriate
sidebar_due_date_spec.rb sidebar_milestones_spec.rb sidebar_subscription_spec.rb sidebar_time_tracking_spec.rb	- newly split from sidebar_spec.rb - changed the title of the spec to be more descriptive - replaced let! with let_it_be where appropriate

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 there is no user-facing change.
• 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