A11y: Heading Structure in Activity > Edit

[mercime]

WP 4.4-trunk, BP 2.4-trunk

Legend:
(x) to be changed in another patch
(-) revised
(o) no change

Headings:
(x) <h2> Editing Activity -> <h1> Editing Activity in #6581
(-) <h3> Action -> <h2> Action
(-) <h3> Content -> <h2> Content
(o) <h2> Status
(o) <h2> Primary Item/Secondary Item
(o) <h2> Link
(o) <h2> Type
(o) <h2> Author ID

Patch attached.

[DJPaul]

@mercime you can commit this now for 2.5; thanks.

[mercime]

In 10359:

A11y: Adjust heading structure in Activity > Edit.

Fixes #6699. See #6581.