modal-lg support small screen.

modal-lg size support small screen size.

current
---
```css
@media (min-width: 768px) {
  .modal-lg {
    width: 900px;
  }
}
```
- When small screen size, the modal width is larger than window width , scrollbars are displayed. 

this pull request
---
```css
@media (min-width: 768px) {
  .modal-lg {
    width: 100%;
  }
}
@media (min-width: 992px) {
  .modal-lg {
    width: 900px;
  }
}

```
- When small screen size, the modal width is 100% , scrollbars are not displayed.