Model refactor, part 2

[DingDongSoLong4]

This is some more refactoring and minor improvements, following on from my earlier model refactoring. I'm trying to be a bit better with sending through smaller PRs, so there's more to come - expect a part 3 :)

The commit messages are for the most part fairly self-expanatory:

• 4bec851 adds a bunch of conversion methods to the changesetTranslator, which allows for a lot of duplicated hasField checking and type conversions to be moved out of the various resolver functions.
• f74252b ensures that every error resulting from a conversion is wrapped in a suitable message - an invalid ID will no longer just give a strconv.Atoi: ... error, but rather converting id: strconv.Atoi: ....
• 8b33f95 adds New and NewPartial functions and ensures they are used everywhere, to avoid forgetting to set/update CreatedAt and UpdatedAt. While making this change, I indeed found, and fixed, a few places where this had been forgotten.
• 27016aa replaces getStashIDsForStudio and getStashIDsForPerformer with calling the GetStashIDs method directly - this is cleaner and reduces overhead vs LoadStashIDs.
• e6d865d just fixes a few tests broken by 8b33f95.
• c594f7d removes ImageCreateInput in favour of just passing in the fileIDs directly to Create. This is consistent with how Create works for both scenes and galleries.

[WithoutPants]

Just some minor stuff.

[WithoutPants]

I've rewritten the assert functions into internal matching functions for the moment. The existing functions were sometimes returning false, and sometimes asserting and returning false and an assert was incorrect for a matching function. We can move them into a public location if the need arises.