filter spv index per semester (filters on previous filter)

app/views/semester_process_volunteers/_index_nav.html.slim

@@ -2,4 +2,5 @@ nav.navbar.section-navigation
   hr
   ul.list-inline
     li= button_link 'Neuen Semester Prozess erstellen', new_semester_process_path, dimension: 'sm'
+    li= render 'semester_filter'
   hr

app/views/semester_process_volunteers/_semester_filter.html.slim

+- semester_filter = params[:semester].presence
+.btn-group
+  button.btn.btn-default.dropdown-toggle type='button' data-toggle='dropdown' aria-label="Semester" aria-haspopup='true' aria-expanded='false' class="btn-md"
+    = "Semester: "
+    = semester_filter ? params[:semester] : @semester
+    = ' '
+    span.caret aria-hidden='true'
+  ul.dropdown-menu
+    li
+    -@semester.collection.each do |(name, code)|
+      li= link_to name[0..15], request_params_filter(semester: code)