Add PR title to PR number in Changelog

[EoinShaughnessy]

This PR adds a previously absent PR title ('Avoid invalid nesting of <span> within a <dd> in summary list') to the Frontend Changelog.

[36degrees]

It also adds some standard wording ("This was added") before the fix link at the end of a release note.

I think 'this was added' makes sense when we're talking about new features, but I'm not sure it quite works for breaking changes – we've not really added anything? We're just asking the user to make a change.

[EoinShaughnessy]

@36degrees Ah gotcha - thanks. That makes sense. Will revert.

[vanitabarrett]

@EoinShaughnessy Should we do something similar for:

This was added in pull request 1963: Remove deprecated $govuk-border-width-form-element-error variable.

It's a bit confusing to say we added a removal of something?

[EoinShaughnessy]

@vanitabarrett Hmmm yeah, good point. Should we change the title of #1963 to 'Avoid deprecated $govuk-border-width-form-element-error variable'? Do we also need to say where users should avoid the variable?