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

_includes/components/sidebar.html

@@ -1,7 +1,7 @@
 <div class="side-bar">
   <div class="site-header" role="banner">
     <a href="{{ '/' | relative_url }}" class="site-title lh-tight">{% include title.html %}</a>
-    <button id="menu-button" class="site-button" aria-label="Open menu">
+    <button id="menu-button" class="site-button btn-reset" aria-label="Toggle menu" aria-pressed="false">
       <svg viewBox="0 0 24 24" class="icon" aria-hidden="true"><use xlink:href="#svg-menu"></use></svg>
     </a>
   </div>
@@ -38,7 +38,9 @@
               <ul class="nav-list nav-category-list">
                 <li class="nav-list-item{% if page.collection == collection_key %} active{% endif %}">
                   {%- if collection.size > 0 -%}
-                  <a href="#" class="nav-list-expander"><svg viewBox="0 0 24 24"><use xlink:href="#svg-arrow-right"></use></svg></a>
+                  <button class="nav-list-expander btn-reset" aria-label="Toggle collection {{ collection_value.name }}" aria-pressed="{% if page.collection == collection_key %}true{% else %}false{% endif %}">
+                    <svg viewBox="0 0 24 24" aria-hidden="true"><use xlink:href="#svg-arrow-right"></use></svg>
+                  </button>
                   {%- endif -%}
                   <div class="nav-category">{{ collection_value.name }}</div>
                   {% include nav.html pages=collection key=collection_key %}