Only show discussion topic search cancel button when appropriate
Only show discussion topic search cancel button when appropriate

Right now we're always showing a cancel button. This changes it to only
show if there is something to cancel and matches usage elsewhere in iOS.

adoosii/edx-app-ios,Ben21hao/edx-app-ios-enterprise-new,appsembler/edx-app-ios,keyeMyria/edx-app-ios,edx/edx-app-ios,Ben21hao/edx-app-ios-new,Ben21hao/edx-app-ios-new,proversity-org/edx-app-ios,Ben21hao/edx-app-ios-new,proversity-org/edx-app-ios,adoosii/edx-app-ios,chinlam91/edx-app-ios,knehez/edx-app-ios,Ben21hao/edx-app-ios-enterprise-new,Ben21hao/edx-app-ios-enterprise-new,nagyistoce/edx-app-ios,yrchen/edx-app-ios,appsembler/edx-app-ios,edx/edx-app-ios,proversity-org/edx-app-ios,yrchen/edx-app-ios,edx/edx-app-ios,chinlam91/edx-app-ios,proversity-org/edx-app-ios,keyeMyria/edx-app-ios,knehez/edx-app-ios,keyeMyria/edx-app-ios,appsembler/edx-app-ios,lovehhf/edx-app-ios,nagyistoce/edx-app-ios,appsembler/edx-app-ios,chinlam91/edx-app-ios,edx/edx-app-ios,nagyistoce/edx-app-ios,Ben21hao/edx-app-ios-new,appsembler/edx-app-ios,Ben21hao/edx-app-ios-enterprise-new,Ben21hao/edx-app-ios-enterprise-new,adoosii/edx-app-ios,proversity-org/edx-app-ios,knehez/edx-app-ios,edx/edx-app-ios,adoosii/edx-app-ios,knehez/edx-app-ios,proversity-org/edx-app-ios,yrchen/edx-app-ios,nagyistoce/edx-app-ios,edx/edx-app-ios,keyeMyria/edx-app-ios,nagyistoce/edx-app-ios,knehez/edx-app-ios,lovehhf/edx-app-ios,lovehhf/edx-app-ios,edx/edx-app-ios,Ben21hao/edx-app-ios-new,lovehhf/edx-app-ios,chinlam91/edx-app-ios