Remove flask-bootstrap dependency

The official flask-bootstrap is still not compatible with bootstrap 4.
The application basically only uses the main template and one macro.
It's easier to include directly bootstrap in CSEntry.
It makes upgrading to the latest bootstrap much easier and removes a
dependency.

Update to bootstrap 4.1.3

Added:
- jquery-3.3.1.min.js
- bootstrap.bundle.min.js (include popper but not jquery)

Note that jquery.slim can't be used.
We need for example $.getJSON for datatables.