isochrones too opaque by default / tedious to adjust all isochrones manually

I like to be able to see the map under the isochrones in order to easily answer questions such as "Staring at X, where could I walk to in 30 minutes, 1 hour, 2 hours, etc." However, with multiple stacked isochrones at 0.3 opacity, the map is almost completely obscured. In order to see anything I generally have to carefully drag each isochrone opacity down to 0.1.

The opacity resets to 0.3 every time the isochrones are regenerated (e.g. when you change the reach centre.)

Things which I believe could improve the situation:
* A single slider that changes the opacity of all the isochrones (As suggested in Issue #358)
* Remember the user's last used isochrone opacity
* Add an option to the options section to choose the isochrone opacity before generating
* Change the default to be less opaque (though this might not please everyone)

No files or tests are named. Start by locating the isochrone rendering and opacity controls, then inspect how regeneration resets opacity; confirm whether the intended outcome is a shared slider, persistence, a pre-generation option, or a lower default, and verify that the selected behavior does not unexpectedly reset the user's choice.

Written by the indexing model from the issue text.