Configuration UI, proxy improvements

Features

• Moved configuration to a new slack/slack-config.xml to make it easier to update
• Introduced Administration UI for configuration instead of having to use the XML
• Test your Slack configuration without having to trigger a build using the new Test Notification button.
• Support for Incoming WebHook URL instead of api token.

Bug Fixes

• Fixed proxy configuration thanks to @uldissturms
• Fixed link for TC9 documentation. Thanks @mfloryan