MDL-68919 theme_boost: prevent nav-tabs collapsing on safari

- Flexbox seems to have an issue on safari which shows when using
the activity chooser on mobile. this might be Flexbug moodle#14 listed
on https://github.com/philipwalton/flexbugs

course/templates/activitychooser.mustache

@@ -41,7 +41,7 @@
                     {{>core_course/local/activitychooser/search}}
                 </div>
                 <div data-region="chooser-container" class="chooser-container">
-                    <div class="nav nav-tabs z-index-1" id="activities-{{uniqid}}" role="tablist">
+                    <div class="nav nav-tabs flex-shrink-0 z-index-1" id="activities-{{uniqid}}" role="tablist">
                         <a class="nav-item nav-link {{#favouritesFirst}}active{{/favouritesFirst}} {{^favourites}}d-none{{/favourites}}"
                            id="starred-tab-{{uniqid}}"
                            data-toggle="tab"