Restructure issue list template, styles (#25750)

Restructure issue list template, styles (#25750)

This PR does various modifications on the issue list shared template:
- restructure layout to achieve better responsiveness
-  fix various style issues
- restructure styles (better result with less code :)
- remove numerous `gt-*` patches and other unneeded classes -> use
existing css classes

<details>
<summary>Before:</summary>

![Bildschirmfoto vom 2023-07-07
14-35-00](https://github.com/go-gitea/gitea/assets/47871822/5301fc80-d58b-41c2-8090-5867e2e8459c)
![Bildschirmfoto vom 2023-07-07
14-35-19](https://github.com/go-gitea/gitea/assets/47871822/d36ab691-7dc4-44bb-a8f6-a4105a2a6777)
![Bildschirmfoto vom 2023-07-07
14-35-43](https://github.com/go-gitea/gitea/assets/47871822/0762c852-eb40-4b00-8757-c5dbfd2ae569)
</details>

<details>
<summary>After:</summary>

![Bildschirmfoto vom 2023-07-07
14-32-04](https://github.com/go-gitea/gitea/assets/47871822/0ac42d36-f98f-451a-b556-70c913d19df8)
![Bildschirmfoto vom 2023-07-07
14-31-32](https://github.com/go-gitea/gitea/assets/47871822/520fac9c-33e3-4290-bfe1-6e941f1100f8)
![Bildschirmfoto vom 2023-07-07
14-31-14](https://github.com/go-gitea/gitea/assets/47871822/05ad46bb-3c67-403d-85b8-d12d64f99c59)

</details>

---------

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