A11y: New List Table headings in wp-admin Activity

[mercime]

WP 4.4-trunk, BP 2.5-trunk.

As outlined in ​make.wordpress.org/core, new headings have been added to improve accessibility for screens with List Tables.

Legend:
(o) changed in #6581
(+) add new heading
(-) revised heading level

Headings:
(o) <h1> Activity <h1>
(+) <h2> Filter activity list </h2> (screen reader only)
(+) <h2> Activity list navigation </h2> (screen reader only)
(+) <h2> Activity list </h2> (screen reader only)
(-) <h5> Reply to Activity </h3> bump to <h3> Reply to Activity </h3>

Screenshot is attached to show the new headings and new heading level for reply to activity section.

[mercime]

6741-add.patch adds accessible hidden headings and text for BP_Activity_List_Table.
6741-bump.patch bumps heading level for the BP_Activity_List_Table Reply Form.

[mercime]

Updated patch fixes errors in WP 3.9.

[mercime]

In 10463:

A11y: Add headings for screen readers in the Activity list table panel.

See #6741.

[mercime]

In 10464:

A11y: Bump heading of the Reply form in the Activity list table.

Fixes #6741.