Touch up settings: alignment, spacing, error states

Fixes https://github.com/vector-im/riot-web/issues/10554

Issues fixed:
* Fields were not ~30px from the avatar (too much right margin)
* Tooltips overflowed the dialog on some resolutions
* SetIdServer didn't have an error state for making the field red
* Spacing between sections in Discovery was wrong (fixed by just removing the problematic n+2 selector - it didn't help anything)

res/css/_common.scss

@@ -561,5 +561,10 @@ input[type=text]:focus, input[type=password]:focus, textarea:focus {
 }
 
 @define-mixin mx_Settings_fullWidthField {
-    margin-right: 200px;
+    margin-right: 100px;
 }
+
+@define-mixin mx_Settings_tooltip {
+    // So it fits in the space provided by the page
+    max-width: 120px;
+}