Update the sign up page and box layout to new branding

[joepavitt]

Description

• Update "Box" layout to new branding
• Shift the "FlowFuse" logo to left if there is "splash" content, otherwise keep central above the sign up form.
• Update Login Page details to new branding
• Update "Create Account" specific styling too

Sign Up - Without Splash

Sign Up - With Splash

Login Screen

Related Issue(s)

Closes #4880

Notes

As per the designs linked in #4880, there may be further iterations of the 'login' page, but no need to hold up this iteration.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.63%. Comparing base (bdf5dae) to head (ec67163).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4924   +/-   ##
=======================================
  Coverage   78.63%   78.63%           
=======================================
  Files         324      324           
  Lines       15265    15265           
  Branches     3507     3507           
=======================================
  Hits        12003    12003           
  Misses       3262     3262           

Flag	Coverage Δ
backend	78.63% <ø> (ø)

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