Displays Warning Message if Cookies Not Enabled

[ckilcrease]

This addresses #2337

Adds a check for cookies using navigator.cookiesEnabled (tested in Chrome 64.0.3282.186, works both when cookies are blocked altogether and when only blocked for localhost:8888). If the user doesn't have cookies enabled, a request to enable cookies appears in body of modal as follows:

[takluyver]

Thanks, this is looking good. A couple of things:

• Could we ask you to do this with your editor configured not to strip trailing spaces? If you look at the changes, you'll see that >100 lines are changed because of automatically removed whitespace. This can cause unnecessary merge conflicts, and it makes git blame less useful.
• Having two buttons seems redundant; the user doesn't really have two options. Maybe get rid of the 'OK' button in this case?
• I think it would be good for the message to mention that Jupyter needs cookies to work. Also, if we can find a good website explaining what cookies are and how to make sure they're enabled on different browsers, we could link to it.

[ckilcrease]

Definitely - we will get started on those changes! 👍

[ckilcrease]

Made most of the changes discussed, but I wanted to check in re: websites on cookies before choosing one; I think MDN does a good job of explaining them concisely, although parts of the page go into details that might not really concern users (such as creating cookies, scope, etc.) - if this would be an issue, I can find one that only presents the overview/relevant details. (Also, accidentally clicked "close and comment" earlier - apologies for that!)

[takluyver]

Thanks!

I like MDN, but I think it's too technical for this. If people want help enabling cookies, they're probably going to be put off by all the detail. I was thinking more of pages like this one or this one. (Those are just the first ones I found with a quick search; there may well be better ones out there).

[ckilcrease]

That makes sense - I looked at a few other articles as well, but it seems like the WikiHow one is the most accurate (many seem to be outdated) and easy to follow, so at the moment I have that one linked as follows:

[takluyver]

Thanks @ckilcrease and @rosaswaby :-)