[ticket/12532] Add navbar_header_username_append/prepend

PHPBB3-12532
[ticket/12532] Add navbar_header_logged_out_content event

PHPBB3-12532
[ticket/12532] Fix tab -> space indentation in events.md

PHPBB3-12532
[ticket/12532] Move logged_out event into its own <li>

PHPBB3-12532
Merge pull request #2640 from dhruvgoel92/ticket/12532

[ticket/12532] Add navbar_header_username_append/prepend

* dhruvgoel92/ticket/12532:
  [ticket/12532] Move logged_out event into its own <li>
  [ticket/12532] Fix tab -> space indentation in events.md
  [ticket/12532] Add navbar_header_logged_out_content event
  [ticket/12532] Add navbar_header_username_append/prepend