Various optimizations

[hirasso]

Description

• add a trailing slash to more local links (hope I caught them all now)
• normalize/update a few other URLs (hash links, absolute URLs...)
• Wording: "Create a Plugin/Theme"
• Fixed a js bug for ads with no image

Checks

• [x] The PR is submitted to the master branch
• [x] The code was linted before pushing (npm run lint)
• [x] All tests are passing (npm run test)

Additional information

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