Since we validate the file existence ourselves, no need to have
AsyncWebServer do it again.

Useful for checking that I haven't broken anything.

Remove the large stack buffer as we're just going to copy it in to a
heap buffer anyways.  Later we can refine the length estimation or use a
rope-style dynamic data structure like DynamicBufferList.

Use a static Print* to transform old oappend calls to print calls.

Reduce the total number of calls by using printf_P and skipping atoi().

Use named functions to describe what's being printed.

Use a name that makes it a bit clearer what the output is.  The new name
is applied consistently through most uses.
Usermods are not yet updated.