feat(mobile): add the number of activities to the activity button label

i18n/en.json

@@ -10,6 +10,7 @@
   "active_count": "Active: {count}",
   "activity": "Activity",
   "activity_changed": "Activity is {enabled, select, true {enabled} other {disabled}}",
+  "activity_count": "{count, plural, =0 {Activity} =1 {1 activity} other {{count} activities}}",
   "add": "Add",
   "add_a_description": "Add a description",
   "add_a_location": "Add a location",

i18n/ja.json

@@ -9,6 +9,7 @@
   "active": "アクティブ",
   "activity": "アクティビティ",
   "activity_changed": "アクティビティは{enabled, select, true {有効} other {無効}}になりました",
+  "activity_count": "{count, plural, =0 {コメント} other {コメント\n{count}件}}",
   "add": "追加",
   "add_a_description": "説明を追加",
   "add_a_location": "場所を追加",