Fix attribute name for active class in init_page.js

[matthewbbrandt]

When adding styles to a sort nav, the active nav element can be selected using .components--sort-nav a[className="js-selected"]. However, it seems the intention is to make a class attribute so that the active element can be selected with .components--sort-nav a.js-selected.

Other notes

• I did not investigate any downstream impacts.
• Assigning a className attribute also occurs in jinja2/user_profile/user_edit.html. This might also be worth reviewing.