Open issues page: Order by priority image button has non-descriptive label

Steps to reproduce:

1. Navigate to and activate "Projects" link from the banner region and select any project.
2. Navigate to and activate the "Issues" link from the left navigation menu.
3. With screen reader running in background, navigate to the "Order by Priority" image button present in main region.

Screenshot:

Actual behaviour:

The current sorted state of the order (ascending/ descending) is not known to the screen reader users as it is easily understood by sighted users looking at the up/down arrow image icon.

Expected behaviour:

The current state of order should be announced for the screen reader users along with the label of the sort image button when it receives focus. For eg: "Order by priority, sorted ascending" OR "Order priority in descending order".

AXQA Ref:AXQA-117