Bug: Publicly shared board links not working

[wuwinson]

Steps to reproduce the behavior

1. Open the Share dialog on a board as an admin
2. On the bottom of the dialog, click on the "Publish" tab
3. Toggle on the "Publish to the web" button
4. Copy and paste link into a browser
5. See error - issue only occurs Community, but not on our bug bash server.

Expected behavior

Link directs user to publicly shared board with read-only access.

Screenshots (optional)

Community:

https://community.mattermost.com/team/qghzt68dq7bopgqamcnziq69ao/shared/b4ctmm917q3rzdfi451qtuxjzcw/vsyng4ghehbgsmp7gxfxk81bxxa?r=kfsmtwmhwtpug6xnou7swweo1dw

Bug bash v76:

https://boards-bugbash76.test.mattermost.cloud/plugins/focalboard/team/7qgkkun8kjb4jbjqo435gd63nh/shared/bf6gc56y8wig9tnp6ocescsj59h/vf9cn41xbj3bf7d8wnsoeybhsxh?r=ko33u7s8nq33syn6e7trq7f9dhh

Edition and Platform

• Edition: Mattermost Boards (plugin)
• Version: v7.7
• Browser and OS: Chrome on Mac

[ogi-m]

@wuwinson This also reproduces on cloud test servers now that they're running in product mode.

cc @sbishel

[wiggin77]

During the bug bash this issue was reported as also involving old/existing share URLs having the plugins/boards prefix. I don't think our share links ever had that prefix (unless I'm missing something). While the link generator needs to be fixed, do we really need the server to convert/redirect any old URLs due to MPA?

[lindy65]

Tested on this PR and this issue is fixed 👍