handle validation of ALE

[nicoalee]

At the moment, the user cannot determine whether or not they are using ALE, or how to even use it.
In the meta-analysis specification step, I want to be able to select ALE and then see a checkbox option for "use sample size". Below this option should be text describing how neurosynth compose will first look at annotations, then analysis level metadata, and then study level metadata for sample size.

On checking the checkbox, validation should be run on the studyset to determine if each study included in the studyset has a corresponding sample_size. This can be at the annotation/analysis/study level. If no sample size, then an error alert message should appear showing links to the studies that are missing sample size.