Restore visible focus indicator for keyboard tab navigation (#6938)

Closes #6065.

The blanket `:focus` rule in `public/assets/css/main.css` strips
`outline` and `box-shadow` from anchors, inputs, textareas, buttons,
and the Bootstrap `.btn` family. With both indicators gone, keyboard
users lose all visual feedback while tabbing through forms (the
case-contact form was called out specifically) -- a serious WCAG 2.4.7
"Focus Visible" failure for an app whose users include CASA volunteers
and supervisors who rely on keyboard navigation.

Scope the existing reset to `:focus:not(:focus-visible)` so mouse
click-focus stays clean, and add a sibling `:focus-visible` rule that
draws a 2px outline in the project blue (#4a6cf7) with 2px offset.
Browser support for `:focus-visible` is universal at this point
(Chrome 86+, Firefox 85+, Safari 15.4+); pre-`:focus-visible` browsers
silently fall back to the still-present `:focus` rule.

The `.btn.focus` and `.btn:active`/`.btn.active` selectors are left
on the reset rule because they represent intentional Bootstrap state
classes, not the `:focus` pseudo-class -- they should not gain a focus
ring just for being active.

Other `outline: none` rules (`.select-style-{1,2,3} .select-position
select:focus` at lines 1226/1286/1340) are scoped to selects that get
a `border-color` change as their focus indicator, so they still
communicate focus and are out of scope here.

Tested locally: tabbing through `/cases/.../case_contacts/new` now
shows the blue outline on every input and the submit button; mouse
clicks do not.

Author: mvanhorn

public/assets/css/main.css

@@ -20,11 +20,11 @@ body {
   -moz-box-sizing: border-box;
   box-sizing: border-box; }
 
-a:focus,
-input:focus,
-textarea:focus,
-button:focus,
-.btn:focus,
+a:focus:not(:focus-visible),
+input:focus:not(:focus-visible),
+textarea:focus:not(:focus-visible),
+button:focus:not(:focus-visible),
+.btn:focus:not(:focus-visible),
 .btn.focus,
 .btn:not(:disabled):not(.disabled).active,
 .btn:not(:disabled):not(.disabled):active {
@@ -34,6 +34,14 @@ button:focus,
   -moz-box-shadow: none;
   box-shadow: none; }
 
+a:focus-visible,
+input:focus-visible,
+textarea:focus-visible,
+button:focus-visible,
+.btn:focus-visible {
+  outline: 2px solid #4a6cf7;
+  outline-offset: 2px; }
+
 a:hover {
   color: #4a6cf7; }