Handle weird version strings in download counter for archive.org

[HebaruSan]

Problem

The download counter throws this now:

InternetArchive id not found in downloads result: KerbalInfinity-1-0,2 - Pyrox
Traceback (most recent call last):
  File "/home/netkan/.local/lib/python3.10/site-packages/netkan/download_counter.py", line 206, in get_result
    counts[ckan_ident] = counts.get(ckan_ident, 0) + result[ia_ident]['all_time']
KeyError: 'KerbalInfinity-1-0,2 - Pyrox'

Cause

When the mirrorer uploads that mod, the space in the version string is replaced with an underscore to conform to archive.org's rules about identifier formatting (see #307 and #328). But when the download counter looks for it, it's not.

• https://archive.org/details/KerbalInfinity-1-02_-_Pyrox

Changes

• Now mirror_item is migrated from CkanMirror to Ckan along with its dependencies
• Now we use mirror_item in the download counter to ensure consistent item ids