Support plugin and Theme compatibility version manifests (#9995)

Adds a new rake task `plugin:checkout_compatible_all` and
`plugin:checkout_compatible[plugin-name]` that check out compatible plugin
versions.

Supports a .discourse-compatibility file in the root of plugins and themes that
list out a plugin's compatibility with certain discourse versions:

eg: .discourse-compatibility
```
2.5.0.beta6: some-git-hash
2.4.4.beta4: some-git-tag
2.2.0: git-reference
```

This ensures older Discourse installs are able to find and install older
versions of plugins without intervention, through the manifest only.

It iterates through the versions in descending order. If the current Discourse
version matches an item in the manifest, it checks out the listed plugin target.
If the Discourse version is greater than an item in the manifest, it checks out
the next highest version listed in the manifest.

If no versions match, it makes no change.

spec/components/version_spec.rb

@@ -46,4 +46,61 @@ describe Discourse::VERSION do
     end
 
   end
+
+  context "compatible_resource" do
+    after do
+      # Cleanup versions
+      ::Discourse::VERSION::STRING = [::Discourse::VERSION::MAJOR, ::Discourse::VERSION::MINOR, ::Discourse::VERSION::TINY, ::Discourse::VERSION::PRE].compact.join('.')
+    end
+
+    shared_examples "test compatible resource" do
+      it "returns nil when the current version is above all pinned versions" do
+        ::Discourse::VERSION::STRING = "2.6.0"
+        expect(Discourse.find_compatible_resource(version_list)).to be_nil
+      end
+
+      it "returns the correct version if matches exactly" do
+        ::Discourse::VERSION::STRING = "2.5.0.beta4"
+        expect(Discourse.find_compatible_resource(version_list)).to eq("twofivebetafour")
+      end
+
+      it "returns the closest matching version" do
+        ::Discourse::VERSION::STRING = "2.4.6.beta12"
+        expect(Discourse.find_compatible_resource(version_list)).to eq("twofivebetatwo")
+      end
+
+      it "returns the lowest version possible when using an older version" do
+        ::Discourse::VERSION::STRING = "1.4.6.beta12"
+        expect(Discourse.find_compatible_resource(version_list)).to eq("twofourtwobetaone")
+      end
+    end
+
+    it "returns nil when nil" do
+      expect(Discourse.find_compatible_resource(nil)).to be_nil
+    end
+
+    context "with a regular compatible list" do
+      let(:version_list) { <<~VERSION_LIST
+        2.5.0.beta6: twofivebetasix
+        2.5.0.beta4: twofivebetafour
+        2.5.0.beta2: twofivebetatwo
+        2.4.4.beta6: twofourfourbetasix
+        2.4.2.beta1: twofourtwobetaone
+        VERSION_LIST
+      }
+      include_examples "test compatible resource"
+    end
+
+    context "handle a compatible resource out of order" do
+      let(:version_list) { <<~VERSION_LIST
+        2.4.2.beta1: twofourtwobetaone
+        2.5.0.beta4: twofivebetafour
+        2.5.0.beta6: twofivebetasix
+        2.5.0.beta2: twofivebetatwo
+        2.4.4.beta6: twofourfourbetasix
+        VERSION_LIST
+      }
+      include_examples "test compatible resource"
+    end
+  end
 end