Accessibility: Add headings for screen readers in template files

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Headings give structure to webpages, showing the relative importance of each section of the content to screen reader and other assistive technology users.

The majority of the BuddyPress screens only contain the <h1> heading with the title of the component as well as <h2>s around member usernames and group admins for example, for the respective components. We will be adding headings for screen reader and AT users similar to how we added accessible headings to our BP admin screens e.g. List tables for ​Activity, ​Groups, and ​Pending Users.

Example:

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Activity headings for Sitewide, Group, Member streams

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Headings for all Group > Manage screens are added/moved above respective forms.

[mercime]

In 11005:

Accessibility: Add headings in Groups-Manage screens.

Headings for screen readers are added in all the Group management
screens except for the Cover Image heading which has been bumped up
and moved above and outside its form.

See #7216.

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Screen reader headings for directory screens

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Screen reader headings for friends screens

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Screen reader headings for group screens

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Screen reader headings for member screens

[mercime]

Screen reader headings patches attached for directory, member, group, and friends screens.
Need to change 7216-activity-streams.patch uploaded previously.

[mercime]

In 11129:

Accessibility: Add headings for screen readers in template files.

See #7216.

[mercime]

In 11166:

Add translator comments for screen-reader headings.

See #7216.