Made use of "kB" vs. "KiB" consistent

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Continuation of #10661.

This PR makes the usage of "kB" and "KiB" consistent across the project. The notations equal the internationally recognized ones:

Short form | Long form | Value in Bytes
--- | --- | ---
B | bytes | 1
kB | kilobytes | 1000
KiB | kibibytes | 1024
MB | megabytes | 1000000
MiB | mebibytes | 1048576
GB | gigabytes | 1000000000
GiB | gibibytes | 1073741824

The reason for differentiating between the two is that several users got confused with the current mix (see #10648, #10917, #10661).

FileValidator, UploadedFile and the ProgressBar helper were changed accordingly.

Follow-up feature request: #10962