server/perkeepd/ui: Drag-and-drop upload files and directories

[omarchehab98]

Summary:

Re-implement drag-and-drop handler to upload files and optionally support directories if browser implements webkitGetAsEntry on DataTransferItem.

Test Plan:

Case A) Directory

1. Drag and drop a directory
2. Perkeep should receive all files, even deeply nested ones

Case B) Files

1. Drag and drop a selection of many files
2. Perkeep should receive all files

Case C) Other elements

1. Drag and drop non-file and non-directory elements such as an HTML image
2. Nothing should happen

Closes: #54 #657

[omarchehab98]

In the second commit, I added abort handlers to server connection methods so that we can implement cancel functionality in the upload dialog. I should have made this it's own pull request tbh, I'll rebase it onto master and remove it from this PR

[omarchehab98]

@bradfitz @mpl could someone please review this?

[mpl]

I've just had a quick try, and I see that this PR "flattens" all the directories hierarchy, i.e. it uploads each file as the camliContent of a stand-alone permanode. it's good, but I don't remember what we said last time we discussed it (if we ever), so I'm not sure this is what we want as the default behaviour.

I can see at least 2 other alternatives:

1. we'd still have a permanode per file, but we'd keep the hierarchy through sets, i.e. each directory would be represented by a set (a permanode with a camliMember attribute).
2. we do not create a permanode per file. we faithfully create files and directories schemas, just as things were on the host.

So I think we need to discuss that on the referenced issue before moving forward with that PR.