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.

+ yaml mode

Open source icon set with ready-to-use fonts for bootstrap
See https://useiconic.com/open/

- update flask-bootstrap
- update datatables to v1.10.15 for bootstrap 4 support