add sortable table headers in POI list view

[jonbulz]

Short description

Add sort function to POI list view

Proposed changes

• allow sorting by Title, Street, Postal Code, Country and Category
• because the list view features the 'Publication status', the standard render_table_header helper function could not be used. instead, the sort_link tag is used directly in the sortable table headers

Side effects

• Because the table header cannot be rendered directly from the table_fields property, it needs to be ensured that every change in the table headers in the template is also reflected in the table_fields

Faithfulness to issue description and design

There are no intended deviations from the issue and design.

How to test

• go to POI list view
• check the sorting works for Title, Street, Postal Code, Country and Category

Resolved issues

Fixes: #1936

---

Pull Request Review Guidelines

[MizukiTemma]

Thank you for the PR 😸 Generally it looks good. I found one thing :eyes

I tested with those 4 POIs

Desc sorting seems to work good

but asc sorting shows three times "Test-Ort" somehow.

Could you check once again?

💡 and I think we can add a release note in this PR, as the changes are visible for users.

[jonbulz]

@MizukiTemma thanks for your review! I thought I had that already covered, but it turns out the way Django ORM works in this respect is more complex than I thought. I learned something new from that 😄
Anyway, I think I fixed it now :)

[MizukiTemma]

Thank you for updates 👍 Works as expected, the codes look also nice 😸