Accessibility: Bump headings in Groups > Manage Members screen

[mercime]

Basic rationale: see #6871.

Manage Members screen will be structured as follows:
1) in #7216, new -> <h2 class="bp-screen-reader-text>Manage Members</h2>
2) <h4> to <h3> headings for Admins, Mods, and Member Headings
3) in #7105, <h5> headings for each group member will be replaced by @dcavins

Patch and screenshots for 2) above coming up.

[mercime]

In 11006:

Accessibility: Bump headings in Groups > Manage Members screen.

Fixes #7217.