Allow the site to load even if the external CSS does not

[jyn514]

Our external CSS is only used for the header bar and a few other things - the site will look ugly without, but the rustdoc pages should still be usable.

This also changes several <script> tags to be inside the body, since
it's invalid for them to come after:

Partially addresses #979, cc @Byron. I can set up a staging server in a little bit so you can verify this actually helps.

[Byron]

I am happy to help :)! Thanks so much!

[jyn514]

Since #981 is waiting on some design decisions, I'm thinking about this more to fix the immediate issues.

putting the css in the body might cause some flicker in the screen, which imo is worse

@pietroalbini I actually think it's better to allow loading to fail, if you have a slow connection and the CSS times out you should still be able to see the rest of the site.

Here's loading with 'Good 2G' (whatever firefox thinks that is): https://youtu.be/tfXVmckH_j4
With 'Wifi', it still looks pretty good - there's a noticable load time, but no flicker: https://youtu.be/MOAJ53C8EMY
With the CSS in the head as currently deployed, it looks about the same, I didn't see a noticeable difference.

Without any external CSS at all (I just commented it out) it looks pretty bad but you can at least see the documentation:

@Byron what do you think about those last two images? Is that usable or not?

[Byron]

@jyn514 Thanks a lot for pushing for an intermediate solution - I can't wait to have docs.rs back in any condition. Since the proposed solution doesn't seem to have negative impact even for slow connections, it seems implementing this won't hurt in the meanwhile.

I also tried again and realized that after three minutes when the browser decides to give up, docs.rs will load just fine (if enough CSS is cached).

[Byron]

It looks like this one is ready to go, and I along with the entirety of China would greatly benefit from this PR being merged.

Is that correct, or is there anything else impeding it?

[jyn514]

Closing this; it's no longer necessary after #981