Rename "domain" to "coding list"

Even though we ourselves have gotten used to it, the term "pseudonym domain" has often caused confusion outside of the castellum team. Only recently we realized that this is basically a synonym for "coding list". So it seems appropriate to change our wording.

This is quite a big change, including some breaking API changes:

• CASTELLUM_STUDY_DOMAIN_BITS and CASTELLUM_SESSION_DOMAIN_BITS have been renamed to CASTELLUM_STUDY_PSEUDONYM_BITS / CASTELLUM_SESSION_PSEUDONYM_BITS
• /domains/ in API URLs was replaced by /coding-lists/ (e.g. /execution/api/studies/5/coding-lists/6f7e93b8-4ddd-4438-9140-4628043e83fc/)
• The response object for /execution/api/studies/5/coding-lists/ has the format {"coding_lists": […]} instead of {"domains": […]}