sections and tags page workflow

Came up while investigating for https://github.com/guardian/workflow/issues/1443

We get a runtime error from:
https://workflow.code.dev-gutools.co.uk/admin/sections-and-tags

this seems to be a bug in the code that parses the config.capiKey value and uses it to construct a Json object to pass to the page template. On fixing that locally and adding a capi-key to local conf, the page itself seems to be broken - request to add tags to a section return 403 errors.

have asked CP about the functionality on Prod - as yet, no-one remebers using it or needing it.

It seems this page is redundant - sections and tags are now handled by tag manager.

## solution
We should double check that this page has no useful functionality, but if not, we should remove it and the supporting code for it.

This may remove a significant amount if dead code.

Start at the /admin/sections-and-tags page and trace the config.capiKey parsing, page template, and supporting code mentioned in the issue. Check with the Tag Manager functionality whether this page has any remaining useful behavior. Done means confirming redundancy and removing the page and supporting code if it is no longer needed.

Written by the indexing model from the issue text.