allow for more than 20 chars in target_id

[nickreich]

The FluSight project wanted to use "wk flu hosp rate change" which is 23 characters long. The limit appears to be 20. Can we increase this to 30, and maybe throw a warning if >20?

[annakrystalli]

I'll have to code up the > 20 warning in R but can do if you feel it's necessary.

Happy to change the max to 30.

Also moving this issue to the schemas repo.