Accessibility: Change markup order of `<tbody>` and `<tfooter>` in BP List Tables

[mercime]

By default, the focus order for keyboard users and screen readers follows the structure of the underlying HTML markup. By moving <tbody> before <tfooter> within the List Tables of our Activity and Groups screens:

• screen readers will read the table rows in the same order that these are rendered visually in the front end, instead of hearing the same information in the top row and last row being read before the second row.
• keyboard users won't be disoriented when the focus suddenly jumps to the last row (tfooter) after tabbing through first row(thead) of the List Table and then focus jumps up back to the second row (tbody).

Keyboard tabbing, before patch:

Keyboard tabbing, after patch:

Screen reader, before and after patch:

Patch coming up.

[dcavins]

Just a note: With HTML4, <tfoot> had to appear before the <tbody>; that requirement has been relaxed with HTML5: ​https://developer.mozilla.org/en-US/docs/Web/HTML/Element/tfoot

[mercime]

Thanks @dcavins! We are fortunate that the WordPress admin has been using HTML5 DOCTYPE declaration for a while now so we're good to go here for a11y's sake :)

[mercime]

In 10776:

Accessibility: Improve focus order in the Activity and Groups screens.

Ensured that the underlying DOM order is consistent with the
visual order by moving <tbody> before <tfoot> within the List
Tables in the Groups and Activity wp-admin screens.

Fixes #7065.