Feature Request: Scheduling Archival from the UI

## Type

- [ ] General question or discussion
- [x] Propose a brand new feature
- [ ] Request modification of existing behavior or design

## What is the problem that your feature request solves

Currently scheduling ingestion of new urls requires writing a cron job external to the web UI (external to the docker container in my case) which isn't entirely ideal in a docker/self-contained setup. I believe this would be a nice convenience feature for users that might want to manage the entire operation of AB from within the web UI.

## Describe the ideal specific solution you'd want, and whether it fits into any broader scope of changes

This feature would add a method for setting up scheduled pulls from various data sources via the web UI rather than only externally via cron. I specifically imagine at least a way to specify a RSS feed to be subscribed to that it can watch for new content from (something like Wallabag in my particular imagined use case). Technically I think this would involve a new menu/button in the UI and should dovetail with the internal scheduling processes already available.

## How badly do you want this new feature?

- [ ] It's an urgent deal-breaker, I can't live without it
- [x] It's important to add it in the near-mid term future
- [ ] It would be nice to have eventually

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- [x] I'm willing to contribute dev time / money to fix this issue
- [x] I like ArchiveBox so far / would recommend it to a friend
- [ ] I've had a lot of difficulty getting ArchiveBox set up

Start by examining the web UI and the internal scheduling processes mentioned in the request, then trace how scheduled ingestion currently works through external cron and the Docker setup. Define how RSS or other data-source schedules would be configured and surfaced in the UI. Done means users can create and manage scheduled pulls without an external cron job.

Written by the indexing model from the issue text.