Add gl-button class to app/views/shared/issuable directory

What does this MR do?

Apply gl-button class to buttons in the app/views/shared/issuable directory. This is for issue #2311792 under epic &4001 (closed)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required) not applicable for this change
• Code review guidelines
• Merge request performance guidelines
• Style guides not applicable for this change
• Database guides not applicable for this change
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #231172 (closed)