Fix excessive blank space in project activity view

7075c5a1 introduced a regression in GitLab 11.7 by adding a
`d-sm-block`, which caused the RSS feed icon to wrap to another
line. Switching to `d-sm-flex` appears to make this appear on the same
line.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56225

app/views/projects/_activity.html.haml

 %div{ class: container_class }
-  .nav-block.d-none.d-sm-block.activities
+  .nav-block.d-none.d-sm-flex.activities
     = render 'shared/event_filter'
     .controls
       = link_to project_path(@project, rss_url_options), title: s_("ProjectActivityRSS|Subscribe"), class: 'btn d-none d-sm-inline-block has-tooltip' do