Fix settings panel layout on small screens

Both labels and inputs protruded outside the panel on for example a
phone in portrait mode. This commit fixes that by allowing wrapping and
setting a max-width.

Since the --input-xpadding variable is now used in two different CSS
files, it was moved to constants.css.

app/styles/input.css

@@ -7,10 +7,6 @@
 
 /* ------- SHARED BETWEEN INPUT ELEMENTS -------- */
 
-:root {
-    --input-xpadding: 1em;
-}
-
 input,
 textarea,
 button,