Use native Drupal API for email validation.
Minimize code in use building table row.
Minor punctuation in description and spacing.
Return proper portion of form in ajax callback.
Get the proper list of entity types and exclude mailchimp module entities.
Explicit form validation decleration not required when using standard naming.
String translation handling.
Minor form cleanup.
Clarifying TODOs
Dynamic string translation.
Leave cache time at one day.
Alter array structure in mailchimp_activity_get_all_activity().
Alter activity page callback to account for new structure of activity array and other misc cleanup.