PP-15376 session logout functionality

[hjvoid]

Amending clear session to a full logout.

• This should logout users from Toolbox without logging them out of Github.
• Changed the logout route to use POST instead of GET as per the Passport documentation.
• Changed the anchor tag with an href to a form and button to facilitate a post request.
• This PR will be reverted after staging as there will likely be some issues around the styling of the 'Sign out' button, at the moment we just want to know if the code works as expected in the test environment.
• CodeQL is marking the POST route as missing rate limiting. These changes implement it via pay-js-commons but it looks like CodeQL isn't familiar with custom middleware. Nonetheless, this should be considered in reviewing.

[SandorArpa]

LOKTM