[Enhancement] Allow subtitles in header to include HTML

[troelslenda]

• [ ] I have written a good issue

Describe the enhancement

We would like part of the subtitle to include a link

Describe the solution you'd like

Have you considered any alternatives?

Are there any additional context?

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Checklist:

The following tasks should be carried out in sequence in order to follow the process of contributing correctly.

Refinement

• [ ] Request that the issue is UX refined
• [ ] Request that the issue is tech refined; do not proceed until this is done.

Implementation

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• [ ] Make sure you have read: "Before you get coding".
• [ ] Signal to others you are working on the issue by assigning yourself.
• [ ] Create a branch from the develop branch following our branch naming convention.
• [ ] Publish a WIP implementation to Github as a draft PR and ask for feedback.
• [ ] Make sure you have implemented tests following the guidelines in: "The good: Test".
• [ ] Update the cookbook with examples and showcases.

Review

Once the issue has been implemented and is ready for review:

• [ ] Do a self-review.
• [ ] Create a pull-request. If you created a draft PR during implementation you can just mark that as "ready for review".

[troelslenda]

I might have a different solution. I'll check back before noon.

[troelslenda]

We found a different solution to our issue, in the process I found minor things that could be optimized/adjusted in kirby-header. I will update this issue and corresponding PR later today.