What does this PR aim to accomplish?:

Fix #2445

How does this PR accomplish the above?:

Changing how the Static DHCP Leases are handled.

Currently, clicking on the add or delete buttons triggers a HTML form POST. This action always reloads the page.
Creating an ajax call via javascript will avoid the reload and will allow the use of the utils.showAlert() function (or something similar).
To achieve this, the code to Add and Delete static leases will need to be moved from savesettings.php to api.php.
2 new endpoints will be created on api.php.

Warning:
I didn't have time to finish this PR yet. The current code is incomplete.

Link documentation PRs if any are needed to support this PR:
none

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