Fixes and tweaks for scikit-learn example gallery

[jaquesgrobler]

Recently, the Nilearn website received a good update (Congrats again 👍 )
I did a redo of the docstring-popup display for the examples gallery - which uses no javascript, cleaner and more stable than the current example.

The current implementation of the popup docstrings on the scikit-learn examples gallery looks nice enough but takes a bit of time to initialise due to waiting for other javascripts and can behave buggily if you swoosh the mouse about fast enough.

So I propose to change the scikit-learn examples gallery pop-ups to match that of the nilearn site.
One thing though is that using the exact same implementation as in nilearn means not having the larger thumbnail images appear anymore as @ogrisel mentioned.
Else I could try adding images to the nilearn gallery version.

Let me know what you think.

---

On a side-note, there is some general cleanup of gen_rst that I'll make a PR for anyway. I can either do that separately or together with the above

[rolisz]

Small suggestion for all Javascript files in general: they should be moved to the bottom of the page, so that they don't block page loading and rendering (http://stackoverflow.com/questions/1638670/javascript-at-bottom-top-of-web-page)
Right now, the Google Analytics is at the bottom, but the others are not (Bootstrap, underscore, etc)

[jaquesgrobler]

@rolisz Yeah I spotted that too. That's one of the things that needs to be done too.
Thanks for the input!

[GaelVaroquaux]

By the way, I just sped up doc building significantly with a few lines of
code:

nilearn/nilearn@fb7b80c

[raghavrv]

Fixed in #2604 #2620

@MechCoder could you take a look?

[jnothman]

I don't think that comment is correct, @ragv. This issue does not fix itself.

[raghavrv]

oh.. sorry for that :p I meant #2620 :D

[jnothman]

I don't think this was entirely fixed in #2620, but in #3925, and in the further sphinx-gallery project work. In any case, the issue should probably be closed.

[jnothman]

And thanks for pointing that out!