Migrate GCN Notices email subscriptions

[jracusin]

Acceptance Criteria:

• Parse sites2.cfg for all "DIST_METHOD" that are "LMAIL" or "EMAIL" and active sites (not "NON_ACTIVE_BEGIN_SITE")
• Convert to table format for email notifications
• maintain subscriptions for legacy users like with circulars even if they do not have accounts on new GCN
• load into configuration for legacy users upon first sign in
• inform users of transition and give option to opt out
• disable old sites by making them NON_ACTIVE
• restart gcn classic program to take effect

[jracusin]

Tied to #1089, #1091, #944.