Problem description: When an overlay dialogue is being reloaded, or submitted, in most places in frontend dialogue fields are not disabled. Therefore, when the internet connection is slow, it is possible to modify the fields in the submitted overlay dialogue while it is being loaded.
Once ZBX-16697 is merged, it will also be possible to submit some value from the loading overlay dialogue using keyboard navigation.
It would be awesome if we could disable dialogue elements while it is being loaded, just like it is done in Media type test dialogue.

Steps to reproduce:

1. Navigate to Administration => Media types and press "Test" next to any existing madia type.
2. Once the overlay dialogue is opened, make sure You're using or simulating a slow internet connection and press "Test" button.
3. Notice that while the dialogue is being loaded, all of its fields are disabled and it's not possible to modify them, even through keyboard navigation.
4. Open some other place where configuration is loaded through an overlay dialogue, for example Configuration => Maintenance => Create maintenance period , open "Periods" tab and add a new period.
5. Once the overlay dialogue is opened, make sure You're using or simulating a slow internet connection and press "Add" button.
   Result 1: Notice that the fields of the loading dialogue are still enabled.
6. Attempt to navigate between the fields of the loading dialogue and to modify them.
   Result 2: since the fields of the loading dialogue are not disabled, it is possible to navigate between them using keyboard navigation, and to modify their values.

Expected: from user experience point of view it would be much better if dialogue fields were disabled while the dialogue is being loaded, just like for the Media type test overlay dialogue.

Example: