css: add box-sizing: border-box to base css.

This makes it so that an element's width includes borders and padding,
so that borders or padding don't cause an element to exceed its
specified width. This is a standard part of most CSS resets.

https://developer.mozilla.org/en-US/docs/Web/CSS/box-sizing

app/javascript/src/styles/base/020_base.scss

@@ -1,3 +1,7 @@
+*, ::before, ::after {
+  box-sizing: border-box;
+}
+
 body {
   color: var(--text-color);
   background-color: var(--body-background-color);

app/javascript/src/styles/common/simple_form.scss

@@ -138,7 +138,6 @@ div.ui-dialog {
   textarea, input[type="text"] {
     width: 100%;
     max-width: 100%;
-    box-sizing: border-box;
   }
 
   form.simple_form {

app/javascript/src/styles/specific/common_tooltips.scss

@@ -3,8 +3,6 @@ div[data-tippy-root].tooltip-loading {
 }
 
 .tippy-box[data-theme~="common-tooltip"] {
-  box-sizing: border-box;
-
   border: 1px solid var(--post-tooltip-border-color);
   border-radius: 4px;