allow user to change "originalVolume"-Setting in ui per lang-channel

current state

• key settings of the intrpretation feature can only be set server-wide (html5-config-yml)
• it is not possible to adapt to different audio-levels of different participants / translators

goal

• make some settings available to the user in the html5-ui
• yml-settings could be default, user could change them in an active session without affecting server or other clients

settings:

• origineVolume - for users, per language channel (how much is the original volume attenuated when an interpreter is speaking

how to do it in ui?

• in the left sidebar add a slider above the user list (for participants' point of view)
  • section title "Interpretation Volume"
  • when user has chosen an interpretation channel to listen to, always only one slider visible, when user has chosen "floor" to listen to, show no slider and hide the whole section
  • left end of the slider has the label "original"
  • right end of the slider has the label "interpretation
• the slider position determines the mixing ratio between original and interpreted audio
  • leftmost position: original at 100%, interpretation at 0%
  • rightmost position: original at 0%, interpretation at 100%
  • center: original at 100%, interpretation at 100%
  • values for originalvolume and interpretationvolume is computed from the sliderposition and are updated while moving the slider
• the slider has a linear scale
  • ❓ is this the right choice? we will see 😉
  • (alternative would be a logarithmic scale so that there is more resolution for the user towards the corners and less resolution towards the center)
• initialize the slider value from the config value originalVolume
  • e.g. for the value 0.3 the slider should be initially at a position almost all the way to the right
• keep the value (sliderposition) per language while session is running
• on switching the language initialize the slider with the value for the new language (or hide it completely)