This further reduces the minimum API level to 16, which should encompass
most users stuck on older versions of Android (mainly BlackBerry OS and
Jolla users).
Several issues reported by code analysis were fixed, mainly around issues
with layouts.
added some white icons,
changed hardcoded icons to theme attributes,
changed icon_edit_dark to icon_edit_body to reflect icons position,
grey message bubbles in dark theme,
misc
purged ic_action_chat as it wasn't used
preference use_white_background changed to use_green_background, default true
grey chat bubbles darker, text white
replaced all grey600 with black icons and 0.54 alpha attribute
highlightColor in dark grey chat bubble now darker than background
