362 right to left layout

Added right to left layout option by adding
```
 <html lang="<%=I18n.locale%>" >
```
 Also added rtl.scss for switching styles.
Now we need to add documention which instruct user to add 
```
html[lang=ar] {
  @import "active_admin/rtl.scss";
}
```
Please change to `<html lang="<%= I18n.locale %>">`