Add GUI for specifying kick/ban reason

This replaces the are-you-sure dialog and also shows up when using
/-commands.

Closes: https://github.com/Nheko-Reborn/nheko/issues/239

# Previous commits:
#   e390c398 Allow to specify reason for removed message
#   5949173b Add function to force focus on InputDialog input field
#   9482ac4e Allow explicit selection of SSO method
#   ab05e2d8 Mobile message input (#962)