Improve file validation

[Kevin-CB]

The filepath validation performed on the doCheckLinkIcon needed some improvement, it was still allowing to discover if a directory called userContent[RandomString] existed. Which I agree, is an edge case that will never happen in the real world.

To fix that, I simply added a file separator to the userContent definition to avoid this behavior.

Testing done

Create a directory called userContent2 at the same level as the userContent one and called:

[JENKINS_INSTANCE]/descriptorByName/hudson.plugins.sidebar_link.SidebarLinkPlugin/checkLinkIcon?value=userContent2/[AnotherFile]

I also tested that it was still working for a correct usage, e.g:

[JENKINS_INSTANCE]/descriptorByName/hudson.plugins.sidebar_link.SidebarLinkPlugin/checkLinkIcon?value=userContent/readme.txt