CSSTUDIO-2180: add help page

[cjenkscybercom]

Summary of Changes

• Adds help page (accessible to all users)
• Adds new env variables (REACT_APP_SUPPORT_HREF, REACT_APP_VERSION, REACT_APP_VERSION_HREF), documented in README

Default help shows documentation link leading to the docs here:

Customized help displays links to custom docs, support URL, and version URL (e.g. source code or release notes)

Visual Inspection

• Conformance to Markdown styles: https://olog.esss.lu.se/Olog/help/CommonmarkCheatsheet
  • ...when viewing a log entry
  • ...when previewing HTML while writing a description
  • ...when viewing a log entry in the group view
• Scroll is possible (elements don't overflow their container, and they are scrollable)
  • ...search result list
  • ...log entry group view list
  • ...log entry single view
  • ...create new log entry page
• Overall layout fills full width and height of viewport
• Pagination element doesn't overflow into other elements

[cjenkscybercom]

Please disregard flaky tests; they pass locally.