Allow a custom branch name in settings file

[robertbrignull]

The variant analysis API endpoint accepts a parameter to control the branch of an action used as part of running the queries. This is for internal testing of changes. The option is not available to users who aren't github staff. But it is useful and it'll be more useful if we could set it from the preferences.

I tested it briefly and it seems to be working correctly and passing the value from the preferences file to the API method.

Checklist

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[shati-patel]

Thanks, looks good! Just a few minor comments 💭