make entry form fit width of screen & less high

[alexanderdd]

Steps to reproduce

1. Open announcement center on a smartphone browser

Expected behaviour

Title and body form fields are wider than the screen width.
Announcements are only visible after alot of scrolling down.

Actual behaviour

Title and body form fields fit the screen width, also on smaller screens.
Body form field is only two lines high so you can see the first announcement (below input area) right away, without scrolling.

Nextcloud version: 18.1

App version 3.7.0