Fix registration form accessibility

Currently the Registration Form on the [Events](https://codingblocks.com/events/) page doesn't have any labels. They just rely on placeholders.

Firefox Accessibility checker reports this issue and it is a well known UX practice to keep label.

I can fix the accessibility bug by adding labels and linking them with proper inputs. UI would something like this

打开 Events 页面并检查 Registration Form 的标记，从 Firefox Accessibility checker 识别出的字段开始。为相应的输入添加可见标签，然后在 Firefox 的无障碍检查器中验证表单，并确认现有布局和行为保持不变。

由索引模型根据 Issue 内容生成。