Bug 20941: (follow-up) Switch 'media' -> 'types'

Bug 20941: (follow-up) Switch 'media' -> 'types'

Clearing up the inconsistency mentioned in comment #35. There is one
place where we use the term 'media' for a template variable, everywhere
we refer to material types as 'types'.

NOTE: This is a breaking change for existing backends that still use
'media'. Of the PTFS Europe backends, only the Koha backend uses it,
this will be modified as necessary. Generally backends will supply this
variable themselves, so the breaking-ness of this change should be
minimal.

No test plan as it's backend dependent.

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>