Merge pull request #152 from dyve/master

Master
Linting
Adding form-control-static class to ReadOnlyPasswordHashWidget, fixes #159
Never set required attributes when form.empty_permitted=True, fixes #155
Added test for #155, form.empty_permitted
Added tests and extended bootstrap_button with href parameter to render a button link
Preparing 4.9.0
Merge branch 'release/4.9.0'