Add a limitBitrateByPortalMinimum setting to prevent limiting too much

[robertbryer]

We use the setting limitBitrateByPortal to prevent the player streaming a representation with a resolution that would be too big for the size of the video element. But we have a player that can be resized from quite small to full screen, and we'd like to have some minimum on this setting so we don't fill up the buffer with the very worst resolution in our ABR ladder before maximising.

So, limitBitrateByPortalMinimum takes a value in kbps and prevents limitBitrateByPortal from excluding any representations below this bitrate. The default is 0kbps so all representations can be limited.

            player.updateSettings({
                streaming:{
                    abr: {
                        limitBitrateByPortal: true,
                        limitBitrateByPortalMinimum: 1700
                    }
                }
            });