on the UI when mark as done a already done todo.

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

Use the project finder in the todos finder to limit todos to just ones within projects you have access to.

By joining the Todos on the project table.

A user viewing the TODOs page will see a 404 if there are mentioned labels
in multiple different projects. This is likely a caching bug and only occurs
when Markdown rendering occurs across multiple projects, which is why it's so
tricky to reproduce. This is what I think is happening:

1. LabelReferenceFilter#references_in encounters label ~X for ProjectA and finds the label in the DB as id = 1.
2. LabelReferenceFilter.references_in yields [1, 'X', nil, ...]
3. Since project_ref is nil, AbstractReferenceFilter#project_from_ref_cache caches nil => ProjectA.
4. LabelReferenceFilter#references_in encounters label ~Y for ProjectB and finds the label in the DB as id = 2.
5. LabelReferenceFilter.references_in yields [2, 'Y', nil, ...]
6. AbstractReferenceFilter#project_from_ref_cache lookups nil and returns ProjectA. It was supposed to be ProjectB.
7. A is the wrong project, so the label lookup fails.

This MR caches Markdown references if the key is present.

Closes #17898

We now only create two Todos instead of 21 when testing pagination, and
we've updated the test to be less brittle when dealing with slower CI
environments.