MDL-59696 theme_bootstrapbase: Set max-width for custom-select elements

* Set custom-select elements with a maximum width of 100% of their
container.

theme/bootstrapbase/less/moodle/forms.less

@@ -113,6 +113,15 @@ form {
     margin-right: .3em;
 }
 
+.custom-select {
+    max-width: 100%;
+}
+
+// Make the submit button align with the .custom-select element.
+.custom-select + input[type="submit"] {
+    margin: 0 0 0 5px;
+}
+
 #portfolio-add-button {
     display: inline;
 }

theme/bootstrapbase/style/moodle.css

@@ -13846,6 +13846,12 @@ form {
 .singleselect label {
   margin-right: .3em;
 }
+.custom-select {
+  max-width: 100%;
+}
+.custom-select + input[type="submit"] {
+  margin: 0 0 0 5px;
+}
 #portfolio-add-button {
   display: inline;
 }