Send snippets via email

This would presumably be a user option. The idea is that every week at the deadline -- the same time we send the 'snippets ready' message to slack -- we'd mail you the full day's snippets, rather than making you go to the website.

Start by tracing the existing weekly deadline flow that sends the “snippets ready” Slack message, then identify how user options and the website's daily snippets are represented. Done should include an agreed email preference, weekly delivery of the full day's snippets at the existing deadline, and verification that the message is sent correctly.

Written by the indexing model from the issue text.