Resolve "Clean up CSS for project alerts and flash notifications."

What does this MR do?

Cleans up CSS and html for alert stripes at the top of project pages.

Screenshots

Before:

After:

Are there points in the code the reviewer needs to double check?

In order to get multiple alerts to appear manually while testing, I simply added the following block to app/views/shared/_no_password.html.haml:

.no-password-message.alert.alert-warning.hidden-xs
  You won't be able to pull or push project code via #{gitlab_config.protocol.upcase} until you #{link_to 'set a password', edit_profile_password_path} on your account

  .pull-right
    = link_to "Don't show again", profile_path(user: {hide_no_password: true}), method: :put
    |
    = link_to 'Remind later', '#', class: 'hide-no-password-message'

.no-password-message.alert.alert-warning.hidden-xs
  You won't be able to pull or push project code via #{gitlab_config.protocol.upcase} until you #{link_to 'set a password', edit_profile_password_path} on your account

  .pull-right
    = link_to "Don't show again", profile_path(user: {hide_no_password: true}), method: :put
    |
    = link_to 'Remind later', '#', class: 'hide-no-password-message'

.no-password-message.alert.alert-warning.hidden-xs
  You won't be able to pull or push project code via #{gitlab_config.protocol.upcase} until you #{link_to 'set a password', edit_profile_password_path} on your account

  .pull-right
    = link_to "Don't show again", profile_path(user: {hide_no_password: true}), method: :put
    |
    = link_to 'Remind later', '#', class: 'hide-no-password-message'

Why was this MR needed?

More alert-warning stripes are coming via the CI usage limits implementation in EE (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/965), and these layout margins and colors were becoming messy.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #25743 (closed)