Alert management list spacing

What does this MR do?

A small MVC for #225740 (closed)

In this merge request we remove the extra padding class gl-pl-9 which is not needed and throws off the alignment for both table cells and table header cells: td && th.

Instead we add a new class called sortable-cell which applies the same amount of padding to the table body cells as the header cells which are sortable, thus aligning the content.

Now all the table cells are correctly aligned and scale for all screen sizes: xs <-> xl

Screenshots

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• 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