Refactor i18n and add additional translation keys

[gadenbuie]

I've refactored the internationalization scripts to remove the jquery-i18next dependency in favor of a hand-rolled function that achieves similar results, but lets us store some of the i18n parameters in data-i18n- HTML element attributes.

I also plan to refactor the i18n_translations() function to use a better format and preparation process that will be more robust for Unicode characters not in UTF-8.

The big change here is that localization will now:

• Use the key in the data-i18n attribute of the HTML element to look up the translation
• OR use the key in the data-i18n-attr-<name> attribute to look up the translation for the <name> attribute.
  • e.g. data-i18n-attr-title = "button.runcodetitle" uses the button.runcodetitle key to set the title attribute of the current element.
  • These elements must have a data-i18n attribute for internationalization to occur, if the innerHTML is not to be set, then set data-i18n = "".
• Additional options to be passed to the i18next.t() method can be stored in the data-i18n-opts attribute. This is particularly useful for passing {count: collection.length} for pluralization.

I added an R function helper i18n_span() to help set the above attributes from the R side. For buttons created in JavaScript, these attributes are added as/where needed.

PR task list:

• Update NEWS
• Add tests (if possible)
• Update documentation with devtools::document()

Fixes #476

[schloerke]

LGTM given small code shuffle / notes

[gadenbuie]

LGTM given small code shuffle / notes

Thanks @schloerke! I've updated the comments and shuffled the code as requested 😄