Improve context popup rendering (#30824) (#30829)

Improve context popup rendering (#30824) (#30829)

Backport #30824 by @silverwind

Before, lot of empty space when no labels or body:

<img width="281" alt="Screenshot 2024-05-02 at 13 51 29"
src="https://github.com/go-gitea/gitea/assets/115237/8a980ccd-d53c-43a3-a059-dc8c614621e1">

After, empty space collapsed:

<img width="306" alt="Screenshot 2024-05-02 at 13 51 16"
src="https://github.com/go-gitea/gitea/assets/115237/8d9c154d-5de1-43d0-8536-afd9194d99b3">

All `<p>` (unsuitable) and `<small>` (discouraged in favor of css) tags
are removed.

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