Add loading indicators on first load and when clicking on the button

[RachL]

Description

Loading the DPM is quite slow. We can see this especially on OFN: https://bbb.paquerette.eu/playback/video/f948430dd272495b317fdadb58e108d81e5ea31d-1759395503882/ (start at >3min)

Step to reproduce

1. Login on https://staging.openfoodnetwork.org.uk/
2. Go to Enterprise setting > connected apps
3. See that the DPM can take a while to appear, but worse: when clicking accepting or refusing a few seconds can go by without any change. This often leads the user to click several times

Proposed solution

Add a loading indicator on load and the button after clicking. Alternative for the button: disable the button on first clik