Always use `octicon-eye` on watch button (#30288) (#30307)

Always use `octicon-eye` on watch button (#30288) (#30307)

Backport #30288 by @silverwind

This might appear odd but I think it's the right thing to do: On Github,
the "Watch" button always has the open eye icon:

<img width="177" alt="Screenshot 2024-04-05 at 08 26 48"
src="https://github.com/go-gitea/gitea/assets/115237/0c1188d1-145b-4c6d-909f-2e1460499941">
<img width="179" alt="Screenshot 2024-04-05 at 08 26 40"
src="https://github.com/go-gitea/gitea/assets/115237/e29d91fa-f122-4e10-9589-f79c1d612cf9">

On Gitea, while watching, the icon is this and this sometimes confuses
me slightly, being used to above:

<img width="158" alt="Screenshot 2024-04-05 at 08 29 08"
src="https://github.com/go-gitea/gitea/assets/115237/3301021b-744e-409f-a9d8-887ec2772fdc">

After this PR, both states will use the same icon:

<img width="145" alt="Screenshot 2024-04-05 at 08 26 27"
src="https://github.com/go-gitea/gitea/assets/115237/8addfa5b-c009-4bdb-bfa1-4f3dfaffa4cd">
<img width="161" alt="Screenshot 2024-04-05 at 08 26 33"
src="https://github.com/go-gitea/gitea/assets/115237/cef383e6-2cc0-460f-a4d3-83ebb321debe">

Co-authored-by: silverwind <me@silverwind.io>