MM-61638: Ensure custom buttons provide proper name, role, and state information (#29904)

* MM-61638: Add aria-hidden to icon

* MM-61638: Add type button to sidebar header button

* MM-61638: Fix failing lint

* MM-61638: Add aria attributes in button

---------

Co-authored-by: Harrison Healey <harrisonmhealey@gmail.com>

webapp/channels/src/components/sidebar/sidebar_header/sidebar_header.tsx

@@ -55,9 +55,18 @@ const SidebarHeader = (props: Props) => {
                     title={currentTeam.description ? currentTeam.description : currentTeam.display_name}
                 >
                     <h1 className='sidebarHeader'>
-                        <button className='style--none sidebar-header'>
+                        <button
+                            className='style--none sidebar-header'
+                            type='button'
+                            aria-haspopup='menu'
+                            aria-expanded={menuToggled}
+                            aria-controls='sidebarDropdownMenu'
+                        >
                             <span className='title'>{currentTeam.display_name}</span>
-                            <i className='icon icon-chevron-down'/>
+                            <i
+                                className='icon icon-chevron-down'
+                                aria-hidden={true}
+                            />
                         </button>
                     </h1>
                 </WithTooltip>