Fix external redirect link not shown to author

[ky940819]

Q	A
Fixed Issues?	Fixes #2812
Patch: Bug Fix?	Yes
Minor: New Feature?	No
Major: Breaking Change?	No
Tests Added + Pass?	Yes
Documentation Provided	Yes (code comments)
Any Dependency Changes?	No
License	Apache License, Version 2.0

Fixes an issue where a page configured as a redirect and having a target of anything other than a Page (e.g. Asset, or external URL) does not display the link target to the author viewing the page in edit mode.

This issue occurred because:

1. v1/RedirectItemImpl resolved the Page for the redirect target before handing that page to the LinkManager.
2. LinkManager#get(Page) requires Page to be not null; however it would be passed a null unless the redirect target could be resolved to a Page.
3. In the event that LinkManager#get(null) is called (because the redirect target does not point to a Page), a Link is returned where the URL for the link is null.

Additionally, even if a valid Link were returned, it would not be presented to the author because of a typo in page/v3/redirect.html:

<a [...]>${redirectTarget.page.title || redirectTarget.linkURL}</a>

The problem here is that redirectTarget.linkURL does not exist because there is no method NavigationItem#getLinkURL().
This is probably a simple typo that should have been redirectTarget.link.URL

Solution
The solution included in this PR resolves the issue by:

1. Correcting the redirect.html.
2. Changing v1/RedirectItemImpl to call LinkManager#get(Resource) with a Resource that has the redirect target path as a property, instead of forcing it to be a page and calling LinkManager#get(Page).

This solution allows the link to be anything: Page, Asset, external, etc.
The path/URL will be shown to the author in all cases except where the redirect target is a page, in which case the page title will continue to be shown.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.17%. Comparing base (a5e97cf) to head (8f70fa2).
Report is 4 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2882      +/-   ##
============================================
+ Coverage     87.14%   87.17%   +0.02%     
- Complexity     2698     2703       +5     
============================================
  Files           235      235              
  Lines          7202     7210       +8     
  Branches       1104     1103       -1     
============================================
+ Hits           6276     6285       +9     
  Misses          366      366              
+ Partials        560      559       -1     

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud