Add multiple file uploading

This is an attempt to provide multiple file upload per issue #174.

• The "Add a document" button becomes "Add documents".
• If the user selects only one file in the file chooser, behavior is unchanged.
• If the user shift-clicks or control/command-clicks to select multiple files in the file chooser, each is uploaded.
  • If the user doesn't provide a name or description, behavior for each file is unchanged: the source filename is used and the description is blank.
  • If the user provides a name and selects multiple files, the provided name + a bates number is used as the document name (e.g., if name = "alpha" and multiple files are selected, this would result in alpha_00001, alpha_00002, alpha_00003, ... in the documents list).
    • This is an arbitrary choice, but could be a useful feature to batch rename/anonymize the document names.
  • If the user provides a description and selects multiple files, each uploaded file will use the same description.
• I've updated the localizations, but I'm not a fluent speaker of the other languages, so those should be checked.

This solution makes use of the multiple attribute of the <input type='file' multiple /> element, then iterates over the 1 or more selected files uploading each via a separate XMLHttpRequest, keeping count of how may requests were made and only removing the spinner once every xhr request has either succeeded or errored.