Automated metadata refresh frontend improvements

[tpoot01]

In the PR https://github.com/OpenConext/OpenConext-manage/pull/112 a automated metadata refresh feature was created. Part of this PR was a new tab in the frontend for the SP/IdP entities in which this feature could be configured. Some feedback was provided regarding this frontend, solutions to this feedback has been implemented in this PR:

• The option to enable the automated refresh was enabled even if no metadata URL was configured. This did not cause any problems as entities without metadata URL are skipped in the refresh, but it can lead to confusing situations. Now the check-boxes are disabled and a warning is displayed.
• When the new metadata fields were not present in the schema.json file the tab would still be available but trying to open the tab would lead to errors. Now the tab is no longer available when these fields are missing in the schema.json file.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (611d835) 81.83% compared to head (f656a49) 81.83%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #113 +/- ## ========================================= Coverage 81.83% 81.83% Complexity 1008 1008 ========================================= Files 101 101 Lines 3160 3160 Branches 311 311 ========================================= Hits 2586 2586 Misses 399 399 Partials 175 175 ```

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