all: Store assets as strings (#6611)

Storing assets as []byte requires every compiled-in asset to be copied
into writable memory at program startup. That currently takes up 1.6MB
per syncthing process. Strings stay in the RODATA section and should be
shared between processes running the same binary.

lib/api/api_test.go

@@ -152,19 +152,19 @@ func TestAssetsDir(t *testing.T) {
 	gw := gzip.NewWriter(buf)
 	gw.Write([]byte("default"))
 	gw.Close()
-	def := buf.Bytes()
+	def := buf.String()
 
 	buf = new(bytes.Buffer)
 	gw = gzip.NewWriter(buf)
 	gw.Write([]byte("foo"))
 	gw.Close()
-	foo := buf.Bytes()
+	foo := buf.String()
 
 	e := &staticsServer{
 		theme:    "foo",
 		mut:      sync.NewRWMutex(),
 		assetDir: "testdata",
-		assets: map[string][]byte{
+		assets: map[string]string{
 			"foo/a":     foo, // overridden in foo/a
 			"foo/b":     foo,
 			"default/a": def, // overridden in default/a (but foo/a takes precedence)