Make the angularjs date picker calendar button more accessible

[Jane Sandberg]

Currently, there is no text in this element that would give a screen reader user any indication of what the button does.

We should do the following to make this button more usable:

* Add an aria-label to the button element
* Add an aria-pressed attribute to the button element that is true when datePickerIsOpen, false when !datePickerIsOpen
* Change the <i> to a <span> for better semantics / consistency with other icons in the Web client.
* Add a title attribute to the <span> to provide a tooltip for sighted mouse users.

Branch forthcoming.

[Jane Sandberg (sandbergja)]

The branch is called: user/sandbergja/lp1796903_accessible_calendar_button
Here is a link: http://git.evergreen-ils.org/?p=working/Evergreen.git;a=shortlog;h=refs/heads/user/sandbergja/lp1796903_accessible_calendar_button

Also note that this commit *only* helps with the button to open the calendar. There are upstream issues with the calendar itself (e.g. https://github.com/angular-ui/bootstrap/issues/5520)

Testing notes from the commit:
    1) After applying this commit, navigate to a place in the Web client
    that includes a date picker (such as the checkin screen).
    2) Hover over the calendar button with your mouse and confirm that a
    tooltip appears.
    3) Verify that the button element has an aria-label attribute.
    4) Verify that the button element has an aria-pressed attribute that
    is false when the date picker is closed; true when it is open.
    5) Use a screen reader (like NVDA or ChromeVox). Verify that when you
    tab over to the date picker, the user is informed of the aria-label
    and aria-pressed values

[Galen Charlton (gmc)]

I've pushed a signoff of Jane's patch along with a suggested followup to

user/gmcharlt/lp1796903_angularjs_datepicker_a11y
https://git.evergreen-ils.org/?p=working/Evergreen.git;a=shortlog;h=refs/heads/user/gmcharlt/lp1796903_angularjs_datepicker_a11y

[Jane Sandberg (sandbergja)]

Thanks, Galen. Signoff branch here: user/sandbergja/lp1796903_angularjs_datepicker_a11y_signedoff

[Galen Charlton (gmc)]

Pushed to master, rel_3_3, rel_3_2, and rel_3_1. Thanks, Jane!