[MRG] Tweak loading messages

[betatim]

Instead of always saying "30s or longer" I think it is better to have a "starting" and "taking longer than usual" message. The change here is to show the "longer than usual" message earlier. I chose the cutoff at 17s because the median launch time is around 15s. 90% of launches complete in under a minute and 99% in around 4minutes. Maybe we can use these stats to build a "progress bar" for launches.

[choldgraf]

I think this is a good idea and I like the language. What do you think about also adding a link to the Binder FAQ about "what affects the launch time" section: https://mybinder.readthedocs.io/en/latest/faq.html#what-factors-influence-how-long-it-takes-a-binder-session-to-start

Something like appending this at the end:

"Curious what affects the length of launches? Check out the Binder documentation."

Could also give something to click and read while they wait.

Feel free to say "lets iterate on that language in a different PR" if you like.

[betatim]

I'd advocate iterating because I think the page is already far too busy with links, unfoldable content (the build log) and potentially a preview (or a 404 because preview doesn't work), etc.

My current suggestion would be to replace the "New to Binder ..." line with a more specific one like you suggested "Curious how to make this faster? ...click here..." when the launch takes more than 20seconds or so:

But I'd do that in a second PR because I don't know how to do it off the top of my head :)

[consideRatio]

I think also we should not have a mentioning of a specific number of seconds, but I think we could perhaps do something like kubespawners progressbar listening on events, but instead we are a bit time dependent.

One thing we could do is utilize the size of the image perhaps.

[betatim]

Shall we merge this and then see how we can improve? I think this PR gets us to a state that is better than what we currently have.