Reuse `getCookie` function and rename variable `xsrfCookies`

[fluegelk]

A seemingly identical getCookie(name) function is currently defined in both canvas_editor/static/js/editor/saveAndLoadHandler.mjs and canvas_editor/static/js/editor/projectOverviewManager.mjs.

• Reuse the function instead of re-implementing it
• Check if xsrfCookies should be renamed for clarity: Isn't it only a xsrf cookie if the name is 'csrftoken', i.e., this function could also be used for any other cookies?