Replace invalid unicode with U+FFFD in `task import`

[djmitche]

The task export from 2.6.2 just sort of blasts out the bytes it finds in *.data, regardless of whether those are valid utf-8 (and thus valid JSON). We can't change behavior of released code, but we can make task import a bit more forgiving, and translate those to U+FFFD (�) in the imported tasks, rather than crashing.

[djmitche]

Blocked on https://github.com/GothenburgBitFactory/libshared/issues/77