Remove route when single pages become unpublished

[JohnNiang]

What type of PR is this?

/kind bug
/area core
/milestone 2.8.x

What this PR does / why we need it:

• Remove route when single pages become unpublished
• Add some unit tests against the change.

Which issue(s) this PR fixes:

Fixes #4309

Special notes for your reviewer:

1. Create a single page with slug name about.html and publish it.
2. Create a static file about.html into static folder${halo.work-dir}/static/.
3. Try to request http://localhost:8090/about.html and check the result.
4. Unpublish the single page and then do the step 3 again.

Does this PR introduce a user-facing change?

修复页面取消发布后访问仍然出现页面未找到错误。

[codecov]

Codecov Report

Merging #4318 (68bdd7e) into main (4f1537b) will increase coverage by 0.35%.
Report is 2 commits behind head on main.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main    #4318      +/-   ##
============================================
+ Coverage     60.61%   60.96%   +0.35%     
+ Complexity     2498     2490       -8     
============================================
  Files           370      368       -2     
  Lines         13048    12907     -141     
  Branches        936      923      -13     
============================================
- Hits           7909     7869      -40     
+ Misses         4679     4585      -94     
+ Partials        460      453       -7     

Files Changed	Coverage Δ
...ava/run/halo/app/theme/router/SinglePageRoute.java	90.19% <100.00%> (+24.88%)	⬆️

... and 22 files with indirect coverage changes

[ruibaby]

/lgtm

[guqing]

/approve
meet expectation 🎉

[f2c-ci-robot]

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