Styled the collapsed state of the left sidepanel

What does this MR do?

Introduces the collapsed state styles for the left side panel in the course of &5647 (closed).

To click-test, enable the :sidebar_refactor feature flag for your user:

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

⚠ This MR also fixes an issue introduced in !62905 (merged) where the template changes were not protected by the feature flag check leaking the avatars size changes to the "flag-off" scenario. ⚠

Screenshots (strongly suggested)

With the flag OFF	With the flag ON

Interaction:

With the flag OFF	With the flag ON
without_flag	with_flag

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.

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.

Related to #322680 (closed)