Resolve "Main navbar is broken in certain viewport widths"

What does this MR do?

Makes the following changes in order to have all the items of the top nav fit within all of the new Bootstrap 4 breakpoints:

• Change GitLab logo to show only on desktop breakpoint (lg and up)
  • Move margin from logo to logo text, saving 8px
    • I don't think the "GitLab" logo type is that critical on tablet devices, feel free to object, though!
• Move the following icon links to the "More" nav dropdown
  • Instance Statistics (ConDev Index)
  • Admin area (if Admin)
  • Sherlock Transactions (if enabled)
• Removed separator between Admin icons
  • 
• Change search input width from 240px to 200px
• Move "Activity", "Milestone", and "Snippets" into "More" menu on lg breakpoints
• Show "More" menu of lg breakpoints

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/49713

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.

Closes #49713 (closed)