Update to Bootstrap 4.1 and Fontawesome 5

[scheinercc]

When playing around the code for #17 and #18 I saw how many little (and big - 'flexbox') CSS feature are missing in BS v3.7. Are there any specific requirements binding the project to v3.7? Otherwise, I can see if I can carve out some time to make a proof-of-concept update for you if you want.

[ysard]

Hello, Indeed the flexbox properties are very powerful, nevertheless there are 2 dependencies in the project which will pose problem for this migration :( : https://github.com/Eonasdan/bootstrap-datetimepicker and to a lesser degree: http://markusslima.github.io/bootstrap-filestyle/1.2.3/

I will look the Fontawesome project to avoid tinkering with some icons.