Generate reply key on first submission

Description

When a source visits /lookup for the first time, a GPG reply key is generated. However, if the source does not submit a message, it is impossible to reply to them, as their account is flagged as pending and not displayed in the JI. To avoid creating keys unnecessarily, the key could be created on submission instead if it does not already exist.

User Research Evidence

Observed behavior of sd instances

User Stories

As an admin, I want to keep a manageable number of source accounts and reply keys