Redirect users away from joplin's admin/pages

## Describe the bug

If you visit https://joplin.herokuapp.com/admin/pages/ you see this view:
Which isn't accurate. How did I get to this page you may wonder? I deleted the page I was editing out of the url,
(`admin/pages/280/edit/#tab-content`, removing `280/edit` for example) and got to this view. We should hide this from everyone. Redirect it to the search view?

## URL

## To Reproduce
Steps to reproduce the behavior
example:
1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

## Current behavior
A clear and concise description of what happened

## Expected behavior
Don't show a view that is confusing or misleading. Redirect to a correct view.

## Screenshots
see above

## Device and browser

## Additional context

Start by reproducing the /admin/pages/ URL and the shortened admin/pages/280/edit/ URL described in the issue. Trace the entry point that serves the admin pages view and confirm the appropriate search destination; done means the misleading view is no longer shown and the redirect is covered by a relevant test if one exists.

Written by the indexing model from the issue text.