This (short) PR builds upon #15028 and makes the file search
case-insensitive.
Previously, having a file named `TestFile.cs` would not be shown if
`test` was typed in the search box.
This now changes the matching function to be case-insensitive (without
affecting the UI).
The matching function, `strSubMatch`, is only used for this feature (it
has been introduced by #15028), meaning that this PR does not affect the
behaviour of any unrelated functionality of Gitea.
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