Add un-encrypted warning icon for messages in encrypted rooms

resources/icons/ui/unlock.png

resources/icons/ui/unlock@2x.png

resources/langs/nheko_de.ts

resources/langs/nheko_el.ts

resources/langs/nheko_en.ts

resources/langs/nheko_fi.ts

resources/langs/nheko_fr.ts

resources/langs/nheko_ja.ts

resources/langs/nheko_nl.ts

resources/langs/nheko_pl.ts

resources/langs/nheko_ru.ts

resources/langs/nheko_zh_CN.ts

resources/qml/EncryptionIndicator.qml

resources/qml/TimelineRow.qml

resources/res.qrc

src/timeline/TimelineModel.cpp

src/timeline/TimelineModel.h