Improve a11y for the dropdown menu

Add role="button" to make the component report itself as a button.

Add tabindex so the menu toggle can receive keyboard focus.

Add keydown handlers for `<Enter>` and `<Space>` so the dropdown menu
toggle responds to keyboard input.

Set the aria-pressed attribute to true if the menu is open, or false if
it is closed.

website/client/components/ui/customMenuDropdown.vue

@@ -3,7 +3,7 @@ A simplified dropdown component that doesn't rely on buttons as toggles  like bo
 -->
 
 <template lang="pug">
-.habitica-menu-dropdown.dropdown(@click="toggleDropdown()", :class="{open: isOpen}")
+.habitica-menu-dropdown.dropdown(@click="toggleDropdown()", @keypress.enter.space.stop.prevent="toggleDropdown()", role="button", tabindex="0", :class="{open: isOpen}", :aria-pressed="isPressed")
   .habitica-menu-dropdown-toggle
     slot(name="dropdown-toggle")
   .dropdown-menu(:class="{'dropdown-menu-right': right}")
@@ -66,6 +66,9 @@ export default {
       if (this.openStatus !== undefined) return this.openStatus === 1 ? true : false;
       return this.isDropdownOpen;
     },
+    isPressed () {
+      return (this.isOpen ? 'true' : 'false');
+    }
   },
   mounted () {
     document.documentElement.addEventListener('click', this._clickOutListener);