Add Mail context information when listing Mail statuses

Add 1 or 2 fields to the DB (in addition to the existing "type" field):

• Summary
• Description (not sure if we need this)

For ex:

• when sending a page by mail, the summary would mention the doc title + reference + possibly the revision. No description
• when sending a template mail, the summary could mention the template doc title + reference + possibly the revision. The description could contain the template's subject and content.