Get screenreaders to read the close button on flash messages
Get screenreaders to read the close button on flash messages

the jquery generated flash messages did not have any text on the
close button. this adds an aria-label which screenreaders
(at least VO + safari, which is all I can test atm) will read

closes LS-1231
flag=none

test plan:
  - do something to cause a flash message to appear
    (I went to /courses/1/users, selected Edit Role
    from a user's kabob menu, killed my canvas server,
    then clicked Update)
  > expect the screenreader to read the message and "Close"

Change-Id: I7e56fc0d29ce88843ab417516730e48d2e4d9365
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/244351
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Jackson Howe <318c10a818c1b1f2ec50cdcb1827b2c6c5ebd7dc@instructure.com>
QA-Review: Robin Kuss <0b206b54367b2de0334896ea45826f0e3194bd35@instructure.com>
Product-Review: Ed Schiebel <0ca8a83847a5889cf05cb219c0ab09f6b646243b@instructure.com>

sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms