scanning and poster improvements (#24)

* first pass at refresh-all-metadata by source

* add version to startup logs

* lock media source during refresh

* fix local media source "name" in collection editor

* optimize scanning so playouts only rebuild when necessary

* support more poster file types

* more scanning improvements; check for missing posters during scans

ErsatzTV.Core.Tests/Fakes/FakeMediaCollectionRepository.cs

@@ -45,7 +45,7 @@ namespace ErsatzTV.Core.Tests.Fakes
 
         public Task Update(SimpleMediaCollection collection) => throw new NotSupportedException();
 
-        public Task InsertOrIgnore(TelevisionMediaCollection collection) => throw new NotSupportedException();
+        public Task<bool> InsertOrIgnore(TelevisionMediaCollection collection) => throw new NotSupportedException();
 
         public Task<Unit> ReplaceItems(int collectionId, List<MediaItem> mediaItems) =>
             throw new NotSupportedException();