🏗️ Make defining new task types easier and DRYer

[rowanseymour]

Currently each task implements JSON deserialization of itself. Pushing that up a level, makes for less repetition, easier to test things. Tasks become structs with a Perform method.

So far I've only ported the two task types to the new way, as proof of concept.

[codecov]

Codecov Report

Merging #354 into master will increase coverage by 0.10%.
The diff coverage is 67.85%.

@@            Coverage Diff             @@
##           master     #354      +/-   ##
==========================================
+ Coverage   54.66%   54.76%   +0.10%     
==========================================
  Files         106      108       +2     
  Lines        8883     8914      +31     
==========================================
+ Hits         4856     4882      +26     
+ Misses       3332     3330       -2     
- Partials      695      702       +7     

Impacted Files	Coverage Δ
queue/queue.go	76.74% <ø> (ø)
tasks/base.go	46.15% <46.15%> (ø)
tasks/groups/populate_dynamic_group.go	72.00% <50.00%> (ø)
tasks/interrupts/interrupt_sessions.go	75.67% <92.30%> (ø)

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