Background

I was working on some new enhancements, while I found the modules/context/context.go is quite messy, so I think it's better to refactor first before other works.

Major Changes

• HTMLString(name string, data interface{}) => RenderToString(name base.TplName, data map[string]interface{}), the first paramter is a TplName, and the old HTMLString naming conflicts with other function names like HTML/PlainText
• TmplLoadTimes is reworked. The old TmplLoadTimes in old HTMLString has no effects because the render doesn't use ctx.Data
• NotFound(title string, err error) and ServerError(title string, err error) related functions are refactored, because the title is not title, it only appears in log.
• use PlainText(string) and PlainTextBytes([]byte) to replace old PlainText([]bytes) and HandleText(string)
• remove unused SignedUserName
• use golang standard type contextKeyType struct{}; var contextKey interface{} = contextKeyType{} to replace var contextKey interface{} = "default_context"
• use ctx.RespHeader() to replace ctx.Header(), because ctx.Header() is not clear whether it is for request or response.