Simplify SystemHookUrlValidator to inherit from PublicUrlValidator
Refactor specs to move out shared examples to be used in both
system hooks and public url validators.

Updating security/webhooks.md to match new behaviour
as well as drying up few specs to extract shared
examples

Many code blocks are 4spaced, and they render in GitLab
without coloring as a result, even though they are
fenced with a language label. If in a list, other items
woll render as being in a code block too, even if not
meant to. This fixes all these issues, and cleans up
minor whitespace issues in /user, /security, /ssh
and /migrate_ci_to_ce docs.

Removing extra blank lines in docs that serve
no purpose, as well as some --- lines that also
are not needed.

Remove unneeded trailing whitespace in lines in
/security /workflow /user docs