Merge branch '1628-multi-select' into 'master'

briar-android/build.gradle

...ject/briar/android/conversation/ConversationActivity.java

...oject/briar/android/conversation/ConversationAdapter.java

...rproject/briar/android/conversation/ConversationItem.java

...r/android/conversation/ConversationItemDetailsLookup.java

...iar/android/conversation/ConversationItemKeyProvider.java

...riar/android/conversation/ConversationItemViewHolder.java

...r/android/conversation/ConversationMessageViewHolder.java

...ar/android/conversation/ConversationNoticeViewHolder.java

...r/android/conversation/ConversationRequestViewHolder.java

...src/main/res/drawable/list_item_background_selectable.xml

...oid/src/main/res/layout/list_item_conversation_msg_in.xml

...main/res/layout/list_item_conversation_msg_in_content.xml

...id/src/main/res/layout/list_item_conversation_msg_out.xml

.../src/main/res/layout/list_item_conversation_notice_in.xml

...src/main/res/layout/list_item_conversation_notice_out.xml

...id/src/main/res/layout/list_item_conversation_request.xml

...ndroid/src/main/res/menu/conversation_message_actions.xml

briar-android/src/main/res/values/color.xml

briar-android/src/main/res/values/strings.xml