Default campaign should be determined automatically

[ragesoss]

Current Behavior:

The default term, which is used for the (logged out) home page and the explore page, is set via /settings and needs to be changed twice a year for Wiki Education Dashboard. We routinely create spring and fall terms for each year, but we must also update the default term whenever it's time to change terms.

Desired Behavior:

The default term should be automatically updated at standard dates each year (with a fallback in case the expected new default term does not exist as a Campaign).

[gabina]

I'm currently working on this issue. The approach will be to create a DefaultCampaignUpdateWorker that will execute at every March, 21st and September, 21st, based on the beginning of the spring and fall in northern hemisphere. Please, double check these dates.

The worker will update the default_campaign setting value to "spring_$current-year" or "fall_$current-year" depending on the current month (March or September) and the current year. If the given campaign doesn't exist, it will set the default campaign to [TBD].