Site Health: Include new tests to check for the ability to upload files.

Several new checks:

* `max_file_uploads`
* `file_uploads`
* `post_max_size`
* `upload_max_filesize`
* `upload_max`
* `max_file_uploads`

In addition, new function `parse_ini_size()` that converts shorthand byte strings to bytes. Useful for size comparisons.

Fixes #50038.
Props dd32, donmhico, JavierCasares, SergeyBiryukov, ayeshrajans, Clorith, ipstenu, sabernhardt, whyisjake.

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