Add "dummy" contexts to two i18np

gettext complained that there's another string with the same text but
not plural and that is bad, so add a dummy context to make gettext happy

xgettext: warning: msgid 'Remove Podcast' is used without plural and with plural.
             ./qml/FeedListDelegate.qml:279: Here is the occurrence without plural.
             ./qml/FeedListPage.qml:249: Here is the occurrence with plural.
          Workaround: If the msgid is a sentence, change the wording of the sentence; otherwise, use contexts for disambiguation.
xgettext: warning: msgid 'Delete Download' is used without plural and with plural.
             ./qml/GenericEntryDelegate.qml:294: Here is the occurrence without plural.
             ./qml/GenericEntryListView.qml:207: Here is the occurrence with plural.
          Workaround: If the msgid is a sentence, change the wording of the sentence; otherwise, use contexts for disambiguation.