Styling the fly-out menus [RUN AS-IF-FOSS]

What does this MR do?

Styles the fly-out menus in the left sidepanel together with updating some templates to match the refctoring and provide the needed CSS classes. The main changes:

• Unified the HTML markup in the templates for the menus and sub-menus
• Added the .has-sub-items CSS class to those topo-level menus that have the sub-items. This is used to style the flyouts differently
• Improved the top offset calculation for the fly-out menus
• Actual styling of the fly-out menus

⚠ To smoke test in the browser, you will need to either enable or disable the feature flag:

➜  gitlab git:(322680-contextual-header-styling) ✗ rails c
...
[1] pry(main)> Feature.enable(:sidebar_refactor, User.find_by_username('root'))

Screenshots (strongly suggested)

	Without the FF	With the FF
Expanded with sub-items
Expanded without sub-items
Collapsed with sub-items
Collapsed without sub-items

Does this MR meet the acceptance criteria?

Conformity

• [-] I have included a changelog entry, or it's not 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 #322680 (closed)