Prevent the addition of conflicting tags in New Validate Beta

[misaugstad]

Brief description of problem/feature

We shouldn't be able to add both 'one button' and 'two buttons' to Pedestrian Signal labels, or both 'alternative route present' and 'no alternate route present' to Missing Curb Ramp labels in the new Validate page. As in Explore, this should be prevented!

While we're at it, we may as well add a check for this when updating the label table on the back end. I've already added a check that we aren't adding duplicate tags, but we should add a check for this and print a warning message to logs if we ever try to add both.