Change button disabling/incomplete error logic across client

[ssciolla]

While working on #270, we've discussed the pros/cons of using button disabling to indicate whether the user still needs to complete something before they can proceed. We seem to have settled on the position that only disabling can be confusing to the user. Alternatively, it is probably more clear to allow all buttons to be clicked and then show error alerts where action needs to be taken.

Since we've adopted this alternative in the PR, it would be best to take this approach everywhere for consistency. The main places where disabling currently happens are with "Select Section" workflow steps, and in the "Merge Sections" feature.

[pushyamig]

@ssciolla As understood from our discussion, this is Icebox issue?

[ssciolla]

@pushyamig, that's your call.