Update details on Namespace storage used card

What does this MR do and why?

This MR finishes the last details of implementing https://gitlab.com/gitlab-org/gitlab/-/issues/378319

The changes include:

• Added the __ import for the search placeholder; don't know how this was working before 🤔
• Add percentage remaining on usage stats card;
• Move card title to the top
  • While here, I moved this part of the template to inside the component, as using a slot is not necessary anymore
• Restored the purchased storage help link we used to have
• Tidy up the code a little bit and reduce some code indirection
• Remove unused utils functions: formatUsageSize, calculateUsedAndRemStorage, parseProjects. I don't know when we stopped using those functions, but since it's dead code, I'm removing it.
• Removed formatSizeAndSplit in favor of numberToHumanSizeSplit or numberToHumanSize. Thanks to the suggestion in !125228 (comment 1456510986)

Screenshots or screen recordings

Repository enforcement

Before	After

Namespace enforcement

Before	After

How to set up and validate locally

The best way to understand the scenarios is to read this MR description: !114960 (merged)

Once you have the data/setup for your group, you can visit Usage Quotas Page:

• Group: https://gdk.test:3443/groups/<your group>/-/usage_quotas#storage-quota-tab
• Profile: https://gdk.test:3443/-/profile/usage_quotas#storage-quota-tab

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.