Make mode placeholder value more intuitive

[iandunn]

Migrated from #33 (comment)

This feels like of unintuitive to me, like from the context I expect mode to be a boolean, but then it's name doesn't describe what it is, so I'm left confused.

I had to look through other parts of the code to realize it's a string that has an empty value until a mode like "all speakers" or "group of speakers" is selected.

I wonder if the code would be more descriptive/self-documenting/obvious if we gave mode an initial value of placeholder, and then changed things like this to read 'placeholder' !== mode ?

[ryelle]

I'm going to close this issue, I don't think we gain much by making this change at this time, but we could think about this for future blocks. Feel free to reopen if you disagree.