Fix #138 - Repository browser break freestyle project save (#139)

Move the existing config.jelly file into the location where it will be
found by Jenkins.  Allows the "GitLab" repository browser to be used
from Freestyle projects as intended.

Also adds top level help for the GitLab repository browser so that it
can be found in the Pipeline Steps reference for the GitLabBrowser.

Not yet ready to add an `@Symbol` annotation because this repository
browser 'GitLab' differs only by case from the similar repository browser
inside the git plugin (called 'gitlab').

Author: MarkEWaite

src/main/resources/io/jenkins/plugins/gitlabbranchsource/GitLabBrowser/config.jelly renamed to src/main/resources/io/jenkins/plugins/gitlabbranchsource/helpers/GitLabBrowser/config.jelly

@@ -0,0 +1,6 @@
+<div>
+  Specify the HTTP URL for this project's GitLab page so that links to changes can be automatically generated by Jenkins.
+  The URL needs to include the owner and project.
+  If the GitLab server is <code>https://gitLab.example.com</code> then the URL for bob's skunkworks
+  project might be <code>https://gitLab.example.com/bob/skunkworks</code>.
+</div>