feat(#332): Prioritize form designer's default language

[latin-panda]

Closes #332

I have verified this PR works in these browsers (latest versions):

• Chrome
• Firefox
• Safari (macOS)
• Safari (iOS)
• Chrome for Android
• Not applicable

What else has been done to verify that this works as intended?

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

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

Do we need any specific form for testing your changes? If so, please attach one.

What's changed

• Refactors validation message in the engine
  • Removes hard-coded error message for "required" and "constraint".
  • Removes "origin" since it wasn't used, as the Vue client can evaluate the validation object to determine how to react and display the UI.
  • Validation message can be null, such as when the form designer did not specify a message.
• Refactors default language in form
  • The form language interface has an isDefault property, which is used to mark when a form design explicitly sets one language as the default.

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Latest commit: 765456e

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[latin-panda]

@garethbowen, this is ready for review :)

[garethbowen]

Nice work! The engine side in particular feels so much cleaner.

I've put some minor suggestions inline.

NB: I haven't tested yet.

[latin-panda]

@garethbowen I've wrapped up addressing the feedback and did an extra round of testing. It’s now ready for another look whenever you have a moment :)

[garethbowen]

Fantastic! Thank you! I can now understand dates :)

I gave it a quick test in Firefox and Chrome but there are so many variables with browser locales it's going to be hard to find everything, but it works on my machine.