conversation: add media gallery

.../java/cx/ring/adapters/ConversationMediaGalleryAdapter.kt

.../src/main/java/cx/ring/fragments/ContactPickerFragment.kt

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

...ain/java/cx/ring/fragments/ConversationGalleryFragment.kt

...n/java/cx/ring/viewholders/ConversationMediaViewHolder.kt

...oid/app/src/main/res/layout/frag_conversation_gallery.xml

ring-android/app/src/main/res/layout/item_media_file.xml

ring-android/app/src/main/res/layout/item_media_image.xml

ring-android/app/src/main/res/layout/item_media_video.xml

ring-android/app/src/main/res/menu/conversation_actions.xml

...ain/kotlin/net/jami/conversation/ConversationPresenter.kt

...lient/src/main/kotlin/net/jami/services/AccountService.kt