Fix ARIA labels for all anchors with href="#"; adds aria-pressed information for toggles (#1262)

This follows up from #1259 and closes #1261. Basically, this PR accomplishes the two items discussed in the issue:

1. for all anchors that are *actually* buttons (i.e., have `href="#"`), I've replaced them with a semantic `<button>`
    - under the hood, I've made a `.btn-reset` class pulling out the reset from #1259, so there's no visual change
2. for anchors that are ["toggle buttons"](https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Roles/button_role#toggle_buttons) (the mobile menu nav, sidebar children/grandchildren toggles), I've added an `aria-pressed` property that is updated as the button is clicked

I've also slightly modified some of the `aria-label`s to make them more consistent. Observe that we *shouldn't* update these as the button is clicked; screen readers use the `aria-pressed` property to add an annotation to each button.

To test this,

- the sidebar children and grandchildren can be done on the deploy preview:
    - open an arbitrary page; observe that the sidebar children/grandchildren dropdown ticks now have a proper `aria-label` and `aria-pressed`, as well as otherwise work as intended
    - toggle one of the buttons; observe the `aria-pressed` role changing as this is done
    - open a grandchild page; observe that the `aria-pressed` has a correct default wrt whether or not the page is active
- the mobile menu can be done on the deploy preview; on a smaller viewport, observe the correct `aria-pressed`
- two features require local changes to test:
    - the `site.search.button` needs to be enabled in the `_config.yml`. To test this, locally clone the repo, change the flag, and observe that the button still works as intended + has no visual regressions.
    - the collections feature is a bit more complicated. To test this, locally clone the repo, add an arbitrary collection and changes to `_config.yml`, and observe the same behaviour for the sidebar children/grandchildren above

_sass/buttons.scss

@@ -111,3 +111,13 @@
 .btn-green {
   @include btn-color($white, $green-100);
 }
+
+.btn-reset {
+  background: none;
+  border: none;
+  margin: 0;
+  text-align: inherit;
+  font: inherit;
+  border-radius: 0;
+  appearance: none;
+}

_sass/layout.scss

@@ -145,13 +145,6 @@
   height: 100%;
   padding: $gutter-spacing-sm;
   align-items: center;
-  background-color: transparent;
-  border: none;
-  margin: 0;
-  text-align: inherit;
-  font: inherit;
-  border-radius: 0;
-  appearance: none;
 }
 
 @include mq(md) {