Add theme asset cache

- Use asset cache in Liquid head/foot tags
- Set 30-day cache header for theme assets

src/MyWebLog/Handlers/Admin.fs

@@ -444,9 +444,11 @@ let updateTheme : HttpHandler = fun next ctx -> task {
         let themeName = themeFile.FileName.Split(".").[0].ToLowerInvariant().Replace (" ", "-")
         // TODO: add restriction for admin theme based on role
         if Regex.IsMatch (themeName, """^[a-z0-9\-]+$""") then
+            let conn   = ctx.Conn
             use stream = new MemoryStream ()
             do! themeFile.CopyToAsync stream
-            do! loadThemeFromZip themeName stream true ctx.Conn
+            do! loadThemeFromZip themeName stream true conn
+            do! ThemeAssetCache.refreshTheme (ThemeId themeName) conn
             do! addMessage ctx { UserMessage.success with message = "Theme updated successfully" }
             return! redirectToGet (WebLog.relativeUrl ctx.WebLog (Permalink "admin/dashboard")) next ctx
         else

src/MyWebLog/Handlers/Routes.fs

@@ -126,6 +126,10 @@ module Asset =
                 let headers = ResponseHeaders ctx.Response.Headers
                 headers.LastModified <- Some (DateTimeOffset asset.updatedOn) |> Option.toNullable
                 headers.ContentType  <- MediaTypeHeaderValue mimeType
+                headers.CacheControl <-
+                    let hdr = CacheControlHeaderValue()
+                    hdr.MaxAge <- Some (TimeSpan.FromDays 30) |> Option.toNullable
+                    hdr
                 return! setBody asset.data next ctx
         | None -> return! Error.notFound next ctx
     }