conversation: add "open" action to files

...p/src/main/java/cx/ring/adapters/ConversationAdapter.java

...src/main/java/cx/ring/fragments/ConversationFragment.java

...oid/app/src/main/res/drawable/baseline_open_in_new_24.xml

...droid/app/src/main/res/menu/conversation_item_actions.xml

ring-android/app/src/main/res/values/strings.xml

...main/java/cx/ring/conversation/ConversationPresenter.java

.../src/main/java/cx/ring/conversation/ConversationView.java