Switching one component of the dataset doesn't preserve others

[tsibley]

For example, going to https://nextstrain.org/flu/seasonal/h3n2/ha/2y and then switching segment to na results in viewing https://nextstrain.org/flu/seasonal/h3n2/na/3y instead of na/2y. Similarly, switching lineage from h3n2 to h1n1pdm resets the selected segment and time resolution. It would be nice for dataset switches to preserve as much of the other selected components as possible (i.e. if they exist for the destination dataset).

[jameshadfield]

Feel free to take this issue if anyone would like to tackle it. I believe that this functionality will be shifted to the client during the implementation of #687.

Some relevant info (using the above ha -> na example) - auspice only sends flu/seasonal/h3n2/na to the server. The server then completes this incomplete path using the default value (3y) as it doesn't know about the previous state.