Fix/merge wiki content into page

[ulferts]

Merges the WikiContent model into the WikiPage model. Presumably, the two were separate to support multiple text version of the same page but that reason was no longer relevant with the introduction of the journals. Nowadays, having the two separated does not grant any benefits while increasing complexity.

https://community.openproject.org/projects/openproject/work_packages/32796

The PR does not attempt to alter the behaviour of application in any way.

For the most part, the changes are just the removals of indirections (referencing the wiki page instead of the wiki page's content). In the frontend however a specific view object (WikiPages::AtVersion) has been introduced. Some views, most notably the show pages allow for browsing through different versions of the text. Instead of having multiple objects referenced in the view, the WikiPages::AtVersion object is passed, representing a wiki page at the version requested by the user. Since this affects the whole page, using ViewComponents has not seemed like an appropriate fit. There is no other view component as of now yet which is why the class is placed under app/helpers.

Limitations existing before have been kept. E.g. the journals only cover changes in the text and not in the title or the parent page. The author field existing both in the page (used to be content) as well as in the journal has also been kept. They arguable are unnecessary since the user reference is kept in the journal anyway.

TODO

• Move code from wiki_content.rb into wiki_page.rb
• Write migration to
  • alter the table structure of both wiki_page as well as rename wiki_content_journals into wiki_page_journals. Does not lead to more attributes being journalized but allows to later include e.g. the title or the parent page in the journalization.
  • migrate the existing data
• Adapt the services
• Adapt the contracts
• Adapt the controller
• Adapt the views  

[dombesz]

Looks like a nice refactoring on the wiki pages, congrats @ulferts !🎉
I left a few findings with screenshots.

[dombesz]

LGTM! 🎉
Feel free to merge once CI is green.

[estan]

@ulferts In attempting to upgrade our instance to 13, we're getting

seeder_1    | PG::UniqueViolation: ERROR:  duplicate key value violates unique constraint "index_journals_on_journable_type_and_journable_id_and_version"
seeder_1    | DETAIL:  Key (journable_type, journable_id, version)=(WikiPage, 1, 1) already exists.

during the db/migrate/20230322135932_merge_wiki_content_into_page.rb migration added in this PR.

(When trying to google this problem I came across an old PR that may or may not be relevant: #9598)

[estan]

@ulferts => https://community.openproject.org/projects/openproject/work_packages/49761

[estan]

Apologies, I see this is likely fixed with 1a640c2 already.

EDIT: Actually no, that commit is in v13.0.0, so that's not a fix, or the fix is not working somehow.