Support linking to "old suites" in the HTML index

This is mainly so that for Ubuntu's asgen we can easily link to the
hints for xenial and yakkety, but could be useful for others too.

data/templates/debian/main.html

@@ -17,6 +17,9 @@
     </p>
 
     <h1>Select a suite</h1>
+    {{#oldsuites}}
+      <h3><a href="{{suite}}/index.html">{{suite}}</a></h3>
+    {{/oldsuites}}
     {{#suites}}
       <h3><a href="{{suite}}/index.html">{{suite}}</a></h3>
     {{/suites}}

data/templates/default/main.html

@@ -17,6 +17,9 @@
     </p>
 
     <h1>Select a suite</h1>
+    {{#oldsuites}}
+      <h3><a href="{{suite}}/index.html">{{suite}}</a></h3>
+    {{/oldsuites}}
     {{#suites}}
       <h3><a href="{{suite}}/index.html">{{suite}}</a></h3>
     {{/suites}}

docs/asgen-config.md

@@ -46,6 +46,7 @@ MetadataType | The type of the resulting AppStream metadata. Can be one of `YAML
 ArchiveRoot | A local URL to the mirror of your archive, containing the dists/ and pool/ directories
 MediaBaseUrl | The http or https URL which should be used in the generated metadata to fetch media like screenshots or icons
 HtmlBaseUrl | The http or https URL to the web location where the HTML hints will be published. (This setting is optional, but recommended)
+Oldsuites | This key exists to support migration from an alternative appstream generator. Given a list of suite names, the output HTML will link to `suitename/index.html`.
 Suites | Suites which should be recognized by the generator. Each suite has the components and architectures which should be searched for metadata as children. See below for more information.
 Features | Disable or enable selected generator features. For a detailed description see below.
 CAInfo | Set the CA certificate bundle file to use for SSL peer verification. If this is not set, the generator will use the system default.

src/asgen/config.d

@@ -95,6 +95,7 @@ class Config
 
     Backend backend;
     Suite[] suites;
+    string[] oldsuites;
     DataType metadataType;
     uint enabledFeatures; // bitfield
 
@@ -331,6 +332,12 @@ class Config
             suites ~= suite;
         }
 
+        if ("Oldsuites" in root.object) {
+            import std.algorithm.iteration : map;
+
+            oldsuites = map!"a.str"(root["Oldsuites"].array).array;
+        }
+
         // Enable features which are default-enabled
         setFeature (GeneratorFeature.PROCESS_DESKTOP, true);
         setFeature (GeneratorFeature.VALIDATE, true);

src/asgen/reportgenerator.d

@@ -758,6 +758,11 @@ public:
             renderPage ("sections_index", format ("%s/index", suite.name), secCtx);
         }
 
+        foreach (suite; conf.oldsuites) {
+            auto sub = context.addSubContext("oldsuites");
+            sub["suite"] = suite;
+        }
+
         renderPage ("main", "index", context);
 
         // copy static data, if present