first pass at improving checkpointing

[nojronatron]

See Issue #40 Improved Checkpointing for detail.

• [x] When posting to the outbox, copy the most recently supplied entry in to a read only field indicating it was the end of the prior batch.
• [x] When posting to the outbox, increment the Message # field and copy the old value into a read-only field.
• [x] Update README.

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[jhmartin]

How about including the message number in end-of-prior-batch? That'll cover the 'what message number did I just send' use case.

[nojronatron]

@jhmartin Form updated to show the last message number with the end of prior batch information.

To conserve screen real estate, the read-only text was shrunk a bit, and the label was changed to "End of prior:". Also, the text for label of count of entries was shrunk. So long as the user saves their data and keeps a browser tab open this feature is supported.

The message number is stored in Save Race Data files so that can be used as an alternate reference if the Winlink Express Outbox isn't handy.

"previousRecord":{"runner":"202","action":"DROP","time":"09:38","dayOfMonth":"25","location":"SB","messageNumber":"2"}