Support icon SVG accessibility

Please, first read the section Accessibility under https://icons.getbootstrap.com/#usage

The question is how to extend the [icon render marcro](https://github.com/helloflask/bootstrap-flask/blob/master/flask_bootstrap/templates/base/utils.html#L13) to support `aria-hidden="true"` and `aria-label="..."`

Suggestion is to add `aria-hidden="true"` when an icon is rendered without title and without description. When a description is given (which is likely more detailed than a title) use the description in `aria-label="..."`. Otherwise, when a title is given, use that in `aria-label="..."`.