Webadmin improvements

- Added "files" folder.
- Created a login_template.html file
- Exported stylesheet in a own file
- Exported favicon
- Stop webadmin if template can't load.
Use a static `std::map` for this mapping:
```cpp
ContentType = GetContentTypeFromFileExt(FileExtension);

...

AString  GetContentTypeFromFileExt(const AString & a_FileExt)
{
  static bool IsInitialized = false;
  static std::map<AString, AString> ContentTypeMap;
  if (!IsInitialized)
  {
    // Initialize the ContentTypeMap:
    ContentTypeMap["png"] = "image/png";
    ContentTypeMap["jpg"] = "image/jpg";
    ...
  }
  return ContentTypeMap[a_FileExt];
}
```