Reduce margins on user settings page, introduce `flex-container` (#26046)

Reduce margins on user settings page, introduce `flex-container` (#26046)

Same as https://github.com/go-gitea/gitea/pull/26026 but for the user
settings page. It introduces a new `flex-container` class and shares it
across both pages.

Before and After:
<img width="1264" alt="Screenshot 2023-07-21 at 19 35 57"
src="https://github.com/go-gitea/gitea/assets/115237/1358dab4-55c0-40ce-a4d5-673099304f3d">
<img width="1269" alt="Screenshot 2023-07-21 at 19 35 42"
src="https://github.com/go-gitea/gitea/assets/115237/34812f6d-dc65-4009-b977-90e03efdc6d1">