Allow "show transmissions" toggle to be defined via URL query

[jameshadfield]

Context

#1147 introduced a toggle to control whether we show transmissions on the map. #1152 allowed the state of this toggle to be set via the JSON. #1154 will make this toggle This toggle is always present, regardless of whether the data is actually present in the tree. If transmissions are not defined by the tree, then the toggle has no real effect.

Description

We want the toggle state to be definable via a URL query, similar to many other aspects of auspice. This flow chart explains how this query state should interact with other variables:

Changes to the toggle state (via user interaction) should also update the query - removing it if the new state matches the default, and adding it if it differs.