Add overwrite warning for duplicate file uploads (#2679)

[bstepanovski]

Fixes #2679

Uploading a file with the same name as an existing file had no warning, so files could be overwritten without users realizing it

• Added an overwrite confirmation before uploads start with single and multi-file messaging plus a "never show again" option stored in localStorage
• Intercepted the upload button click instead of using a pre-upload hook so canceling the warning does not leave uploads in a failed state
• Added system tests covering single file overwrite warnings, multi file overwrite warnings, and disabling the warning

[Bubballoo3]

I'm not sure that I like the idea of stacking the modals on top of one another. It seems like that is a consequence of

Intercepted the upload button click instead of using a pre-upload hook so canceling the warning does not leave uploads in a failed state

But I feel like we should be able to intercept the upload and cancel it without leaving things in a failed state. I could be missing something though.

In any case, the modal itself suffers many of the issues noted in #5068. There are two close buttons, the title is not bolded, and the checkbox is part of the main content, when it seems like it would fit better below the dividing line next to the 'cancel' and 'upload' buttons. Finally, I think the message would be better framed as a question, something like 'The following file/s already exist. Overwrite these files?' and then the options can be 'Cancel' and 'Overwrite'. It also seems like the button to continue should be red, since this is a permanent action.

[Bubballoo3]

But I feel like we should be able to intercept the upload and cancel it without leaving things in a failed state. I could be missing something though.

I see why now, since we show the upload progress in the upload modal. So there is no good way to close that and later continue without flashing the modals back and forth

[Bubballoo3]

It also looks like this doesn't present itself during folder uploads. Since a duplicate folder could overwrite or just plain delete several files at once, this is definitely a case we have to handle