authorize: only redirect for HTML pages

[calebdoxsey]

Summary

Currently when a user is not logged in and they don't have access to a route, we redirect the user through the Login flow. Unfortunately this causes issues for single page applications, because usually the request that will result in the redirect is done via AJAX or Fetch, and Javascript can't handle the HTML redirect.

This PR changes the behavior slightly so we only do the redirect if a request for an HTML page is made. Other types of requests receive a 401 instead, which applications can detect and handle appropriately. (For example Grafana reloads the page and sends the user through the Login flow again)

Related issues

Fixes #2176

Checklist

• reference any related issues
• updated docs
• updated unit tests
• updated UPGRADING.md
• add appropriate tag (improvement / bug / etc)
• ready for review

[codeclimate]

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[github-actions]

The backport to 0-14-0 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-0-14-0 0-14-0
# Navigate to the new working tree
cd .worktrees/backport-0-14-0
# Create a new branch
git switch --create backport-2264-to-0-14-0
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick --mainline 1 9dc90d02d0ac39eae66171c9e2189e9693b35a1f
# Push it to GitHub
git push --set-upstream origin backport-2264-to-0-14-0
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-0-14-0

Then, create a pull request where the base branch is 0-14-0 and the compare/head branch is backport-2264-to-0-14-0.