Check Circulars message size before accepting submission

[lpsinger]

Current behavior

There is an effective limit on the size of GCN Circulars due to the limit of 262144 bytes on the Amazon SES API's TemplateData argument. Circulars that would result in TemplateData arguments larger than this value are accepted, but they are never successfully distributed as emails. Also, they are repeatedly sent as emails because the table stream Lambda can never succeed and is retried indefinitely.

Expected behavior

• Circulars form should prevent user from submitting form if size is exceeded
• Action function should reject Circulars larger than this size

Steps to reproduce

See https://gcn.nasa.gov/circulars/39259.

Environment

N/A