Add opensearch manifest for elixir.bootlin.com

[fstachura]

Closes #216

This adds a static opensearch manifest for elixir.bootlin.com. The manifest only works for latest Linux and statically points to elixir.bootlin.com (self hosted instances will need to manually edit the manifest)

[tleb]

Please add more details to your commit messages. The following are all questions I asked myself while reading your code. I don't see anything to change in the diff, but the commit messages are lacking.

• Where do you specify this is Firefox specific? At least that was my understanding of it. Do we want to do the same for other browsers? Do they even provide such features?
• Why do we need to force a specific content-type from the Apache config?
• Where is a link to the opensearch spec (and/or documentation)?
• Why is the autocomplete response the way it is? Why that return value? With a link to the place in the spec that describes that.

Thanks!

[fstachura]

Where do you specify this is Firefox specific? At least that was my understanding of it. Do we want to do the same for other browsers? Do they even provide such features?

It's not Firefox specific, it's an open standard, other browsers at least used to support it (Chrome and pre-chromium Edge). I believe macOS Safari still supports it.

Why do we need to force a specific content-type from the Apache config?

https://developer.mozilla.org/en-US/docs/Web/XML/Guides/OpenSearch

Your server should serve OpenSearch plugins using Content-Type: application/opensearchdescription+xml.

Where is a link to the opensearch spec (and/or documentation)?
Why is the autocomplete response the way it is? Why that return value? With a link to the place in the spec that describes that.

I added that to commit messages