Improves accessibility of link and fake link elements

[lucascumsille]

Fixes: https://github.com/mysociety/societyworks/issues/4538

The styling is based on the GOVUK links

Pending:

• Test cobrands now that I have added variables for the header there is some tidy up to do, we want cobrands navbars to look just like they did before this PR.
• Get rid of text-decoration on sticky-sidebar sticky-sidebar--help

[skip changelog]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.66%. Comparing base (2402023) to head (f15f3ee).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5184      +/-   ##
==========================================
+ Coverage   82.47%   82.66%   +0.19%     
==========================================
  Files         406      406              
  Lines       31721    32105     +384     
  Branches     5045     5205     +160     
==========================================
+ Hits        26161    26540     +379     
- Misses       4069     4070       +1     
- Partials     1491     1495       +4     

Flag	Coverage Δ
	82.66% <ø> (+0.19%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.