Alias list_name to dataset in entities sheet

[lognaturel]

Closes #652

Why is this the best possible solution? Were any other approaches considered?

The intent is to match the user-facing language of "Entity List". We discussed some alternative column names but decided that list_name has the advantage of being consistent with the choices sheet. The meanings are slightly different but very much related.

I went back and forth on the text of error messages. At first I made them generic ("Invalid name") but then I thought that would be too hard to use. I think using "entity list" in the messages is friendlier. People who have existing forms with dataset are early adopters so hopefully are in a better position to figure out the issue if they run into the message.

I also added on case-sensitive checking of reserved entity property names.

What are the regression risks?

This area is well-tested and the change is small so I don't think there are any.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

getodk/docs#1651

Before submitting this PR, please make sure you have:

• included test cases for core behavior and edge cases in tests
• run nosetests and verified all tests pass
• run black pyxform tests to format code
• verified that any code or assets from external sources are properly credited in comments

[lognaturel]

I've targeted a v1.12.x branch off of the current v1.12.1 release so that we can release this immediately. @lindsay-stevens if you have a chance for a quick look, that'd be great since you have the context but I also recognize I'm catching you Friday afternoon! It should be straightforward for someone else to review if that's better. Thanks!