Update bootstrap from a v4-alpha version to the latest release

Fixed welcome to work with the new release
Fixed checkboxes and radiobuttons in bs4 formstyle

applications/welcome/static/css/web2py-bootstrap4.css

@@ -1,7 +1,4 @@
-ul.navbar-nav.navbar-right {
-    position: absolute;
-    right: 100px;
-}
+
 label, th {
     font-weigth: bold;
     white-space: nowrap;
@@ -318,9 +315,6 @@ td.w2p_fc,
 .web2py_grid table {
     width: 100%;
 }
-input[type=checkbox], input[type=radio] {
-    margin: 4px 4px 0 0;
-}
 
 /* for backward compatbility with pre-font-awesome */
 .icon.plus,.icon.arrowleft,.icon.download,.icon.trash,.icon.pen,.icon.arrowright,.icon.magnifier {
@@ -337,4 +331,4 @@ input[type=checkbox], input[type=radio] {
 .icon.trash:before { content: "\f1f8";}
 .icon.pen:before { content: "\f040";}
 .icon.arrowright:before { content: "\f061";}
-.icon.magnifier:before { content: "\f002";}
+.icon.magnifier:before { content: "\f002";}