Messaging: improve ARIA labels to message drawer

Please test these UI changes on: Clean and Boost themes Supported browsers Setup Log in as an admin Create a course C1 Enrol students s1, s2, s3 and s4 to C1 Create group g1 and enable group messaging Add s1, s2, s3 and s4 to g1 Log in as s1 Send some messages to s2 Send some messages to s3 Mute conversation with s4 Send some messages to g1 Star s2 conversation Testing scenario 1: Cancel button label Log in as s1 Open the message drawer Access to the conversation with s2 from the "Starred" section Select 1 or 2 messages, clicking over them Inspect the HTML code for the x next to "Messages selected: z" CHECK the button with data-action="cancel-edit-mode" has also the aria-label="Cancel message selection" attribute   Testing scenario 2: Action menus labels Log in as s1 Open the message drawer Access to the conversation with s2 from the "Starred" section Click over the actions menu ("...") CHECK "User info" is displayed instead of just "Info" CHECK "Unstar conversation" is displayed, instead of just "Unstar" Click over "Unstar conversation" CHECK "Star conversation" is displayed, instead of just "Star" Inspect the HTML code for the "..." button CHECK the button with id="conversation-actions-menu-button" contains also the  aria-label="Conversation actions menu" attribute Access to the g1 conversation Click over the actions menu ("...") CHECK "Group info" is displayed CHECK "Star conversation" is displayed, instead of just "Star" Inspect the HTML code for the "..." button CHECK the button with id="conversation-actions-menu-button" contains also the  aria-label="Conversation actions menu" attribute   Testing scenario 3: Link user profile Log in as s1 Open the message drawer Access to the conversation with s2 Click over the actions menu ("...") Select "User info" Inspect the HTML code for the user profile image CHECK the img element doesn't contain the   aria-hidden="true" attribute Testing scenario 4: Muted conversation label ATTENTION: This scenario should be tested only in master (mute/unmute actions are not available in 3.6) Log in as s1 Open the message drawer Access to the conversation with s4 Inspect the HTML code for the muted conversation icon next to the s4 name. CHECK the span with data-action="muted-icon-container" has also the aria-label="Muted conversation" attribute