opensearch: Add opensearch manifest

OpenSearch is a standard that allows browsers to discover search engines.
A special XML file with information about the search engine is linked
from its webpage in a link tag.
OpenSearch is supported by Firefox and (maybe) macOS Safari.
It used to be supported by Chromium-based browsers, but that feature was
deprecated.
https://stackoverflow.com/questions/56400952/does-chrome-allow-auto-discovery-of-opensearch

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

OpenSearch specification:
https://github.com/dewitt/opensearch/blob/master/opensearch-1-1-draft-6.md

Author: fstachura

docker/000-default.conf

@@ -8,6 +8,9 @@
 <Directory /usr/local/elixir/static/>
     AllowOverride None
     Require all granted
+    <Files search.xml>
+        ForceType "application/opensearchdescription+xml; charset=utf-8"
+    </Files>
 </Directory>
 <VirtualHost *:80>
     ServerName MY_LOCAL_IP

static/search.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="utf-8"?>
+<OpenSearchDescription xmlns="http://a9.com/-/spec/opensearch/1.1/" xmlns:moz="http://www.mozilla.org/2006/browser/search/">
+  <ShortName>Elixir (Linux)</ShortName>
+  <Description>Elixir Cross Referencer - Linux</Description>
+  <InputEncoding>UTF-8</InputEncoding>
+  <Image width="16" height="16" type="image/x-icon">https://elixir.bootlin.com/static/favicon.ico</Image>
+  <Url rel="results" type="text/html" template="https://elixir.bootlin.com/linux/latest/A/ident/{searchTerms}"/>
+</OpenSearchDescription>

templates/layout.html

@@ -15,6 +15,8 @@
             Elixir Cross Referencer - Explore source code in your browser - Particularly useful for the Linux kernel and other low-level projects in C/C++ (bootloaders, C libraries...)
             {%- endblock %}">
         <meta name="viewport" content="user-scalable=no, initial-scale=1, maximum-scale=1, minimum-scale=1" />
+        <link rel="search" type="application/opensearchdescription+xml" title="Elixir" href="/static/search.xml" />
+
         <link rel="stylesheet" href="/static/style.css?v=15">
 
         <link rel="preload" href="/static/img/2penguins.svg" as="image" />