Add utility mixins gl-border-gray-50 & gl-w-90p

What does this MR do?

This Mr adds two more utility mixins, gl-border-gray-50and gl-w-90p They go inline with similar mixins that already exists in GitLab-UI but there lack has been noticed in this MR.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• [-] GitLab UI's contributing guidlines.
• [-] If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• [-] If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• [-] If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • [-] GitLab: mr_url
  • [-] CustomersDot: mr_url
  • [-] Status Page: mr_url
• [-] Added the ~"component:*" label(s) if applicable.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] Security reports checked/validated by a reviewer from the AppSec team