Copy library asset content before parsing it

[jnm]

…when importing a library asset into a survey.

Asking for a review from @duvld, since we worked on this together, but will give the final 👍👎 to fresh-eyed @magicznyleszek.

Checklist

1. ❓ If you've added code that should be tested, add tests
2. If you've changed APIs, update (or create!) the documentation
3. ❓ Ensure the tests pass
4. Make sure your code lints and you followed our coding style
5. If this is a big feature, make sure to prefix the title with Feature: and add a thorough description for non-dev folk

Description

Fixes bug that made it impossible to add more than one library question to a survey using the form builder if either the survey or the library question had a named translation

Related issues

Fixes #2822

[duvld]

👍 looks good, I cannot produce a null translation from adding questions from the library.
Tried to generate a null translation by adding the following into the form and saving:

• blocks with multiple translations,
• questions with multiple translations,
• questions that have form's default language but not as default,
• different combinations of blocks and groups with translations

The only way I can generate a null translation in the library is through #2821, or adding translated library questions to a form with no named translations (both are beyond the scope here)

In regards to tests, (in a quick search) there's not many that test loadDict() but there's a whole suite that tests inputParser wrt translations: https://github.com/kobotoolbox/kpi/blob/master/test/xlform/inputParser.tests.coffee that are run by travis.

[magicznyleszek]

@jnm I love the comments you've added <3

[jnm]

Thank you, @duvld 👍 I wonder if @dorey could comment on how code like addExternalItemAtPosition(), which bridges the ES6 React stuff with CoffeeScript / Backbone code, ought to be tested.

edit: started discussion at https://chat.kobotoolbox.org/#narrow/stream/4-KoBo-Dev/topic/Tests.20at.20ES6-CoffeeScript.20boundary