add ability to display privacy policy link in menu

build.gradle

...siacs/conversations/ui/ConversationsOverviewFragment.java

.../eu/siacs/conversations/ui/StartConversationActivity.java

src/main/java/eu/siacs/conversations/ui/XmppActivity.java

...ain/java/eu/siacs/conversations/ui/text/FixedURLSpan.java

src/main/res/menu/fragment_conversations_overview.xml

src/main/res/menu/start_conversation.xml

src/main/res/values/strings.xml