feat: slotless page templates

[kaljarv]

The current page templates have some drawbacks, because they always re-render the whole page:

• This prevents nice transitions between pages, and is possibly heavier for the frontend
• This prevents supplying parts of the page in layouts, such as having an outer layout that includes the Logout button for in case of the Candidate App or the correct navigation menu.

The current templates have slots to facilitate placement of elements in different parts of the page. This can, however, be achieved by using a CSS grid layout for the page and use components for the different parts that define in which (named) grid area they are placed.

Additionally, this also allows us to control the order of the elements for screen readers such that the secondary actions on the top-right do not have to appear before the main content in the hierarchy if we so wish.

For the progress indicator provided by the templates can be implemented as a store (or a context).

Also, ensure that CLS issues are resolved.

• See also Define transitions between pages and other interactions within the app #113
• See the draft PR poc/feat: slotless page templates #376 for a PoC.