[FIX] Non root url

[MohamedLEGH]

Cakeshop is only working on the root url aka "/" but the application should work in a subdirectory, for example, "/cakeshop".
If I run the application with the option server.servlet.context-path=/cakeshop , the backend will run at the path "/cakeshop", but the frontend will still assume that the path is "/"

Changes made:

• Relative path for link to manage.html page
• Relative path for the web socket path
• CONTEXT_PATH variable with default value to "/"

[CLAassistant]

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