Server check guard change 7 x

[YanaDePauw]

References

Fixes #4074

Description

After getting rid of the of /server/api request for #3961, one more /server/api check was found in the ServerCheckGuard that will be performed with every load to check if the server is not down.

This can still affect performance when a slower REST API is used.

This PR removes the initial call on load to check if the backend is up. Instead it will only perform this call if a request fails with a 500 or a 0 (Unknown) status. If the server request then fails, a redirect is done to the 500 page.

The additional check and redirect are performed in the request effects but will not hold up the rest of the object handling.

Instructions for Reviewers

To test this feature, set up a local backend and frontend where you can turn the backend on and off.

1. In Angular, test on the initial load that no extra check is performed to the api endpoint to verify that it is running.
2. Verify that the repository still runs as it normally does.
3. Turn off the backend and verify that on the next (non-cached) request you are redirected to a 500 page.
4. Turn the backend on again and click the "return to homepage" button.
5. Verify that you are redirected to the homepage and you can browse the repository again.
6. Turn the backend off again and hard refresh the page
7. Verify that you are redirected to the 500 page again.

Checklist

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