Model changes for Letter

[XanderVertegaal]

• Change representation in Django Admin. Instead of "letter #57" there should be a name / one-line identifier so the team can quickly find/identify new letters, e.g. ('No. 60: Boniface to pope Zacharias in 578')
• Add sender
• Add addressee
• Add date
• Add type (e.g. 'friendship letter', 'consensus letter', 'royal order'). This could be an editable category in its own right?
• Instead of certainty, two new fields: historically_attested (letter features in actual, historical events vs. letter features in story/legend/fairytale) and mentioned: (letter is directly mentioned vs. existence of letter is implied/possible).

[lukavdplas]

I would think that the sender, addressee and date fields can be inferred from the actions/roles associated with the letter?