Support fragments in redirects

[ehuss]

Problem

The redirect table does not support fragments. It would be helpful when section headings are changed to provide a way to redirect to the new location (whether or the same page, or a different page).

Proposed Solution

The redirect table should support # in the source and destination.
This is tricky because AFAIK native HTML does not support that. In my books, I have been using javascript redirects.
This would need to handle both scenarios where the original page still exists, and where it has been removed.
This should also consider any security issues like open redirects.

Notes

cc original issue #430

[KFearsoff]

Seems related: #1760, #1715

While renames are different from permalinks, I feel like permalinks to the heading attributes would cover 80% of the use cases. Let's do that first, maybe?