search overhaul (#87)

* add letter bar with no links

* use lucene for search, add paged search results

* add search index version

* index library_name; rebuild index when folder is missing

* maintain index as local movies change

* fix tests

* maintain index as local shows change

* maintain index as plex movies change

* maintain index as plex shows change

* code cleanup

* add duplicate filter to search

* add links to letter bar

* code cleanup

ErsatzTV.Core.Tests/Fakes/FakeLocalFileSystem.cs

@@ -36,6 +36,8 @@ namespace ErsatzTV.Core.Tests.Fakes
             _folders = allFolders.Distinct().Map(f => new FakeFolderEntry(f)).ToList();
         }
 
+        public Unit EnsureFolderExists(string folder) => Unit.Default;
+
         public DateTime GetLastWriteTime(string path) =>
             Optional(_files.SingleOrDefault(f => f.Path == path))
                 .Map(f => f.LastWriteTime)