Expose CLI commands through web-interface

As a sysadmin
I'd like to be able to run maintenance tasks via the web ui
So that I don't have to log on to the server and remember the correct incantation

As a trusted user
I'd like to be able to run limited maintenance tasks (non-destructive re-index, purge) via the web ui
So that I don't have to bother my sysadmin to do it

As a user who has permission to run maintenance tasks
I'd like to be able to see the status of potentially long running tasks I have initiated
So that I can tell when it is finished and don't accidentally start another

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This would probably require tasks for be executed outside of the usual http flow (as many are long-running tasks) and some way of ensuring that tasks are not run more than once - we don't want to try and start more than one re-index or purge.

No files, tests, or entry points are named. Start by locating the existing CLI maintenance commands and the web interface, then investigate how long-running tasks and permissions are handled. Done means trusted users can run permitted non-destructive tasks, duplicate runs are prevented, and initiators can see task status.

Written by the indexing model from the issue text.