Make wording in security report summaries more consistent

Summary

A lot of summary texts in the security dashboards are inconsistent.

Current	Should be	Reasoning	How to re-produce
Security scanning detected no vulnerabilities	Security scanning detected no new vulnerabilities	The merge request page shows Security scanning detected no vulnerabilities even if vulnerabilities exist (but no new ones).	Create an MR without new vulnerabilities (e.g. https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21398)
TYPE detected x new security vulnerabilities	TYPE detected 5 new vulnerabilities	For more consistency (@andyvolpe approved of this here)
TYPE resulted in error while loading results	TYPE: Loading resulted in an error	Better english
TYPE detected X new vulnerabilities and Y fixed vulnerabilities	TYPE detected X new, and Y fixed vulnerabilities	better readability and translatability

The last one actually reduces the need of 3 states to one:

%{reportType} detected X new vulnerabilities and Y fixed vulnerabilities
%{reportType} detected 1 new vulnerability and Y fixed vulnerabilities
%{reportType} detected X new vulnerabilities and 1 fixed vulnerability

to one case

%{reportType} detected X new, and Y fixed vulnerabilities