Switch from manually configuring cron to pebble events approach when available

[mthaddon]

Bug Description

To fulfil the need for a recurring task (checking for theme changes) we're currently configuring cron in the indico container. As discussed in https://discourse.charmhub.io/t/recurring-task-in-k8s-charms/6644/4 this will be supported natively by pebble events at some point. Once that's available we should switch to it.

To Reproduce

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Environment

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Relevant log output

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Additional context

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[amandahla]

Does the #126 fix this too?

[mthaddon]

Not quite, no. We can't consider this item fixed until pebble has support for configuring recurring events. We've disabled cron in that PR, but we may want to re-enable it at some point once we're confident the process can be automated, but currently we'd only be able to do that via manually configuring cron in the workload container.