MDL-15777 - updated most of the portfolio code to use files api.

MDL-15777 - updated most of the portfolio code to use files api.

Not done:

- forum (the rest of the module isn't using files api yet)
- database mod (touches other parts of the code (ods and excel libs)
- portfolio download plugin (needs discussion with petr about userfiles)

all of these have been disabled in the meantime.