core: fix user profile picture size

Since 28f397574f and the removal of the
`flex-basis: 50px` property from `user_profile_pictures_thumbnails`,
the main picture was always displayed small-ish, at least on Firefox.
Setting back a flex-basis helps getting more consistent behavior once
again.

core/static/user/user_detail.scss

@@ -170,6 +170,7 @@ main {
       align-items: center;
       gap: 20px;
       flex-grow: 1;
+      flex-basis: 14em;
 
       @media (max-width: 960px) {
         flex-direction: row;