Fix various loading states, remove `.loading` class (#29920)
Various code was using fomantic `loading` class which I think got broken
a while ago and rendered only a full circle. Fix those to use
`is-loading`.
Before:
<img width="295" alt="Screenshot 2024-03-19 at 22 56 26"
src="https://github.com/go-gitea/gitea/assets/115237/dbe83395-5db4-4868-90bc-3613866a35f0">
After:
<img width="60" alt="Screenshot 2024-03-19 at 22 54 35"
src="https://github.com/go-gitea/gitea/assets/115237/8ac19b7e-035a-4c6d-850b-53a234ef69c2">
<img width="294" alt="Screenshot 2024-03-19 at 22 54 56"
src="https://github.com/go-gitea/gitea/assets/115237/34e819d7-25f7-43a1-9d48-4a68dcd2b6ad">
<img width="320" alt="Screenshot 2024-03-19 at 22 55 16"
src="https://github.com/go-gitea/gitea/assets/115237/05127544-47ff-4e18-9fd8-c84e44c374f8">
<img width="153" alt="Screenshot 2024-03-19 at 23 01 43"
src="https://github.com/go-gitea/gitea/assets/115237/a33248c6-b11d-40ff-82d8-f5a3d85b55aa">
<img width="1300" alt="Screenshot 2024-03-19 at 23 56 25"
src="https://github.com/go-gitea/gitea/assets/115237/562ca876-b5d5-4295-961e-9d2cdab31ab0">
<img width="136" alt="Screenshot 2024-03-20 at 00 00 38"
src="https://github.com/go-gitea/gitea/assets/115237/44838ac4-67f3-4fec-a8e3-978cc5dbdb72">
