Add and update QA text content matchers

What does this MR do?

• Adds a negatable have_text/have_content matcher to be used instead of Capybara's built-in ones
• Include all custom matchers during RSpec config. This should make it simpler to add more matchers - we just need to put them in qa/spec/support/matchers in the Matchers module.
• Update a few tests that used expect(page).to have_content (I opened an issue to update the rest: #281062).

Fixes #266994 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content