Use lowerCamelCase for the JSON API (fixes #1338)

Replace the current mix of UpperCamelCase und lowerCamelCase with
consistent lowerCamelCase keys for the JSON API. Also adapt the frontend
so it works with the changed API.

Attention: this will break existing consumers of the API.

internal/stats/folder.go

@@ -23,7 +23,7 @@ import (
 )
 
 type FolderStatistics struct {
-	LastFile LastFile
+	LastFile LastFile `json:"lastFile"`
 }
 
 type FolderStatisticsReference struct {
@@ -32,8 +32,8 @@ type FolderStatisticsReference struct {
 }
 
 type LastFile struct {
-	At       time.Time
-	Filename string
+	At       time.Time `json:"at"`
+	Filename string    `json:"filename"`
 }
 
 func NewFolderStatisticsReference(ldb *leveldb.DB, folder string) *FolderStatisticsReference {