[Passage] Mark beginning and end of passage to screenreaders
[Passage] Mark beginning and end of passage to screenreaders

Summary:
Along with the visual designation of the beginning and end of
a passage, we need to have a text designation to make interpreting
the passage using a screen-reader easier.

This diff adds "Beginning of reading passage", "Beginning of
reading passage footnotes", and "End of reading passage" text
for screenreaders.

Fixes T3729

T3729 does not include anything about footnotes, but since those
are also visually distinct I thought it would be important to
textually separate them as well.

When we look into T3733, we might want to make some of these
denotations headers?

Test Plan:
Make a passage widget with a title, text, and footnote, then run

    $(".perseus-sr-only").removeClass("perseus-sr-only");

And verify that the text appears in the right places.

Reviewers: john

Reviewed By: john

Subscribers: jordan, #a11y

Maniphest Tasks: T3729

Differential Revision: https://phabricator.khanacademy.org/D17556

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