Use URL type in most outstanding struct fields (#1468)

Use URL type in most outstanding struct fields (#1468)

* Use URL type in most outstanding struct fields

This fixes all known remaining cases where url fields are stored as
plain strings, with the exception of form fields where empty strings
are used as sentinels (see `diesel_option_overwrite_to_url`).

Tested for regressions in the federated docker setup attempting to
exercise all changed fields, including through apub federation.

Fixes #1385

* Add migration to fix blank-string post.url values to be null

This also then fixes #602

* Address review feedback

- Fixed some unwraps and err message formatting
- Bumped the `url` library to 2.2.1 to fix a bug with serde error
  messages
- Add unit tests for the two diesel option override functions
- Fix migration teardown by adding a no-op

* Rename lemmy_db_queries::Url to lemmy_db_queries::DbUrl

* fix compile error

* box PostOrComment variants