Add a feature and preference to confirm follows. (#4445)

This means a popup will appear if you have that option enabled in the
preferences which will have a popup similar to the unfollow dialog
asking you if you want to follow the user.

app/src/main/java/com/keylesspalace/tusky/settings/SettingsConstants.kt

@@ -71,6 +71,7 @@ object PrefKeys {
     const val SHOW_CARDS_IN_TIMELINES = "showCardsInTimelines"
     const val CONFIRM_REBLOGS = "confirmReblogs"
     const val CONFIRM_FAVOURITES = "confirmFavourites"
+    const val CONFIRM_FOLLOWS = "confirmFollows"
     const val ENABLE_SWIPE_FOR_TABS = "enableSwipeForTabs"
     const val ANIMATE_CUSTOM_EMOJIS = "animateCustomEmojis"
     const val SHOW_STATS_INLINE = "showStatsInline"