Consider switching ActiveJob backend from Sidekiq to SolidQueue

[floehopper]

• SolidQueue would remove our dependency on Redis and thus potentially reduce our monthly costs.
• I've found working with ActiveJob + Sidekiq very confusing. It seems as if ActiveJob itself implements a bunch of stuff that's also implemented in Sidekiq. Also there's a lot of out-of-date Sidekiq documentation around, so it's often hard to work out how to configure stuff.
• SolidQueue is going to be the default in Rails v8.