Block theme: Update pattern workaround to use template filters.

[ryelle]

Fixes #662 — This removes the pattern slug filtering in favor of adding extra custom templates, and filtering to inject those into the hierarchy. This is a more core-native way to handle template overrides, so it will be less likely to break in the future. With this change, we can unpin Gutenberg.

Additionally, this fixes the alignment of the "logged out" error messages.

Screenshots

"Before" here is with GB <18, just to show the alignment change.

Before	After

How to test the changes in this Pull Request:

1. Update locally to Gutenberg 18+
2. Be logged out
3. View the "My patterns" page
4. It should give you a "please log in" message
5. Repeat with Favorites, it should also show a "please log in" message
6. View a pattern you own
7. There should be a status message above the pattern name and pattern actions (edit, etc) under the preview.