Multisite: Show upload space usage for a subsite

[lenasterg]

This pull request fixes the issues descripted in the
Trac ticket: https://core.trac.wordpress.org/ticket/60549

In superadmin 'Edit site' page:

1. Shows the network default Site upload space
2. The site's current use.

and also displays space usage information to the sub site administration in the media gallery pages.

This update introduces a new function, get_space_usage(), which enhances the way WordPress reports upload space usage for a site. The function calculates the space used for uploads and provides a detailed or simplified report based on whether the upload space check is enabled or disabled.

When upload space check is disabled: The function returns a simple message showing the space used in megabytes (e.g., "500 MB Space Used").

When upload space check is enabled: The function provides a more detailed report, including the percentage of space used and the total space allowed (e.g., "Used: 500 MB (50%) of 1 GB").

This enhancement improves transparency and usability for site administrators by offering clearer insights into their site's storage usage. The function also uses localization-friendly strings, ensuring compatibility with multilingual setups.

Technical Details:

The function get_space_usage() leverages existing helper functions like get_space_used(), get_space_allowed(), and size_format() to calculate and format the space usage.

The function respects the upload_space_check_disabled site option to determine the level of detail in the output.

This change is backward-compatible and does not affect existing functionality. It is particularly useful for multisite networks where storage management is critical.
The following images show the visual changes to the various pages.

In "Edit site"

In 'Dashboard'
Before the suggested changes, when the upload_space_check_disabled site option was enabled, the admin had no information about the storage space.

After the suggested changes:

In 'Media Gallery'

In media upload pages

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[github-actions]

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[audrasjb]

I added some commits to fix a handful of WPCS issues.

[audrasjb]

Phew, all the coding standard issues are now fixed 😅