Refactoring relative paths PR#209

[HeinrichAD]

Follow up PR due to some issues and since PR #209 is already merged.

• use html base tag containing the context path once instead inside every single URL/link.
• update and enhance nginx-contextpath example
• export javascript code into separated file
• add TODO note to javascript clipboard check since Firefox and Safari do not support the current implementation from from PR #250

Just a quick question: How do I test the error page? Is there a way to render and show error.jsp?

It would be nice if somebody else would test the changes :sweat_smile:.

[arnaudroques]

Many thanks for your contribution! Right now, we are focusing on the main library rather than the plantuml-server We have to update it to last 1.2023.4 version BTW