Add library `lucene-backward-codecs` to migrate indices.

To be able to read and migrate Lucene indices from old (4.x)
formats to new (5.x) ones, add the `lucene-backward-codecs`
library to the project.
It is added to the `ext` directory and therefore to the classpath.
According to the Lucene documentation, having it in the classpath
can affect performance. But right now the `ext` directory is the
only one available and even for a separate tool for offline
migration the library would be needed.

.classpath

diff --git a/.classpath b/.classpath
index 42cbfbc..5d96d16 100644
--- a/.classpath
+++ b/.classpath
@@ -22,6 +22,7 @@
 	<classpathentry kind="lib" path="ext/wicket-auth-roles-1.4.22.jar" sourcepath="ext/src/wicket-auth-roles-1.4.22.jar" />
 	<classpathentry kind="lib" path="ext/wicket-extensions-1.4.22.jar" sourcepath="ext/src/wicket-extensions-1.4.22.jar" />
 	<classpathentry kind="lib" path="ext/lucene-core-5.5.2.jar" sourcepath="ext/src/lucene-core-5.5.2.jar" />
+	<classpathentry kind="lib" path="ext/lucene-backward-codecs-5.5.2.jar" sourcepath="ext/src/lucene-backward-codecs-5.5.2.jar" />
 	<classpathentry kind="lib" path="ext/lucene-analyzers-common-5.5.2.jar" sourcepath="ext/src/lucene-analyzers-common-5.5.2.jar" />
 	<classpathentry kind="lib" path="ext/lucene-highlighter-5.5.2.jar" sourcepath="ext/src/lucene-highlighter-5.5.2.jar" />
 	<classpathentry kind="lib" path="ext/lucene-memory-5.5.2.jar" sourcepath="ext/src/lucene-memory-5.5.2.jar" />

build.moxie

diff --git a/build.moxie b/build.moxie
index f21241d..2748eab 100644
--- a/build.moxie
+++ b/build.moxie
@@ -145,6 +145,7 @@
 - compile 'org.apache.wicket:wicket-auth-roles:${wicket.version}' :war !org.mockito
 - compile 'org.apache.wicket:wicket-extensions:${wicket.version}' :war !org.mockito
 - compile 'org.apache.lucene:lucene-core:${lucene.version}' :war :fedclient
+- compile 'org.apache.lucene:lucene-backward-codecs:${lucene.version}' :war :fedclient
 - compile 'org.apache.lucene:lucene-analyzers-common:${lucene.version}' :war :fedclient
 - compile 'org.apache.lucene:lucene-highlighter:${lucene.version}' :war :fedclient !org.apache.lucene:lucene-join
 - compile 'org.apache.lucene:lucene-memory:${lucene.version}' :war :fedclient

gitblit.iml

diff --git a/gitblit.iml b/gitblit.iml
index 1f4aa24..25a4447 100644
--- a/gitblit.iml
+++ b/gitblit.iml
@@ -191,6 +191,17 @@
       </library>
     </orderEntry>
     <orderEntry type="module-library">
+      <library name="lucene-backward-codecs-5.5.2.jar">
+        <CLASSES>
+          <root url="jar://$MODULE_DIR$/ext/lucene-backward-codecs-5.5.2.jar!/" />
+        </CLASSES>
+        <JAVADOC />
+        <SOURCES>
+          <root url="jar://$MODULE_DIR$/ext/src/lucene-backward-codecs-5.5.2.jar!/" />
+        </SOURCES>
+      </library>
+    </orderEntry>
+    <orderEntry type="module-library">
       <library name="lucene-analyzers-common-5.5.2.jar">
         <CLASSES>
           <root url="jar://$MODULE_DIR$/ext/lucene-analyzers-common-5.5.2.jar!/" />