Show least CR value on Exchange

**Is your feature request related to a problem? Please describe.**
A margin trader is interested, what CR the lowest CR of all margin position is.

**Describe the solution you'd like**
Show the lowest CR value in the tooltip of the top-bar Collateral Ratio

**Describe alternatives you've considered**
None

Start by locating the top-bar Collateral Ratio tooltip and the existing margin-position collateral-ratio data in the JavaScript/React frontend. Determine how the lowest value across margin positions can be represented there. Done means the tooltip shows the lowest CR value for the user's margin positions and the related UI behavior is verified.

Written by the indexing model from the issue text.