bootstrap4.3.1的datetimepicker没有icon图标

1. cdn

2. html code （FYI: https://eonasdan.github.io/bootstrap-datetimepicker/）

3. reason (bootstrap3.x to bootstrap4.x changelog:  https://v4-alpha.getbootstrap.com/migration/#components)

1. Dropped panels, thumbnails, and wells for a new all-encompassing component, cards.
2. Dropped the Glyphicons icon font. If you need icons, some options are:
    the upstream version of Glyphicons
    Octicons
    Font Awesome
3. Dropped the Affix jQuery plugin. We recommend using a position: sticky polyfill instead. See the HTML5 Please entry for details and specific polyfill recommendations.
    If you were using Affix to apply additional, non-position styles, the polyfills might not support your use case. One option for such uses is the third-party ScrollPos-Styler library.
4. Dropped the pager component as it was essentially slightly customized buttons.
5. Refactored nearly all components to use more un-nested classes instead of children selectors.

4. modify (reference: https://fontawesome.com/v4.7.0/icon/calendar)

5. reference

https://stackoverflow.com/questions/49905458/glyphicon-not-working-in-bootstrap-version-4-1-and-later