Box shadow should be applied to non gl-link links in side nav

What does this MR do and why?

• Fixes the side-nav links to use same styling for focus as it would in the gl-link vue component
  • see: !74551 (comment 736145721) and #346003 (closed)

Screenshots or screen recordings

type	before	after
trials		no change
invite - desktop	no focus ring
invite - mobile	no focus ring
top menu
fly out menu
sub menu
collapsed menu
collapsed menu fly out

How to set up and validate locally

1. Run the following commands in the rails console:

   # Turn on billing features in the app
   ApplicationSetting.first.update(check_namespace_plan: true)
   # Create a new User
   user = FactoryBot.create(:user, username: 'user_288015', email: 'user.288015@example.org', password: '12345678')
   # Create a new Group & assign the new user as its owner
   group = FactoryBot.create(:group)
   group.add_owner(user)
   # Find or create the Ultimate plan
   plan = Plan.create_with(title: 'Ultimate (Formerly Gold)').find_or_create_by(name: :ultimate)
   # Create a new subscription for the Group
   GitlabSubscription.create(namespace: group, hosted_plan: plan, seats: 10, start_date: Date.current, end_date: Date.current.advance(years: 1), trial: true, trial_starts_on: Date.current, trial_ends_on: Date.current.advance(days: 30))
   Feature.enable(:invite_members_in_side_nav)

2. Log into the app with username user_288015 and password 12345678

3. Navigate to Groups › Your Groups › “group1” (or whatever the group was named)

4. Create a project.

5. Check out the feature in the sidebar of the newly created project.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #346003 (closed)