fix(ui): repair stale database list in Log Viewer

[sranka]

Closes #5544

Briefly describe your proposed changes:

• list of namespaces (database + retention policy pair) is fetched whever Log Viewer page is opened
  • it is also verified that the current namespace exists, the default namespace is selected otherwise
• allow clearing local storage with ?clearLocalStorage query, such as `http://localhost:8080/logs?clearLocalStorage
  What was the problem?
• list of databases was not refreshed from the server, local storage was used
  What was the solution?
• fetch the list on open of Log Viewer

• CHANGELOG.md updated with a link to the PR (not the Issue)
• Well-formatted commit messages
• Rebased/mergeable
• Tests pass