Add a setting to configure send option on messaging

This issue should be tested in all platforms (including desktop ones), 1 device per platform. Install the app on the device, login in a site and go to the messaging tab. Click the settings icon in the top right. Check that you see a new setting "Use enter to send". In Android/iOS, check that the setting is disabled by default. In the rest of platforms, check that the setting is enabled by default. Enable the setting if needed and open a conversation. Type some text and click the "Enter" key. Check that the message is sent. In desktop, type some text and click Shift+Enter. Check that a new line is added. Go back to settings and disable the setting. Go back to the discussion, type some text and press Enter. Check that a new line is added and the message isn't sent. In Mac, type something and press Cmd+Enter. Check that the message is sent. In Windows and Linux, type something and press Ctrl+Enter. Check that the message is sent.