It has transpired that on filename-case-sensitive filesystems, the
tournament client does not consistently handle custom asset paths.
Videos and mods could only be looked up from `videos` and `mods`
directories (lowercase), while flags could only be looked up from the
`Flags` directory (uppercase).
A complicating circumstance is that default country flags, coming from
osu-resources, also depend on the flag lookup being uppercased.
To attempt to clean up the handling as much as it appears to be
possible, automatically move user-supplied lowercase directories to
uppercase.
