Rubocop: upgrade and apply

[gonzedge]

Issues found in lib/ and tasks/

1. Explicitly disable Style/ExplicitBlockArgument due to performance hits (0a6aec0)
2. Add missing supers
3. Yoda-style conditionals
4. Bump target ruby required version to current min supported (2.7.0)

Issues found in spec/

1. Use instance_double instead of double
2. Split tests into single-assertion specs when parameterization is possible; disable otherwise
3. Use let! instead of instance variables
4. Remove duplicate test groups
5. Use when at the start of context block descriptions
6. Use described_class, subject in specs where possible
7. Single-line before where possible

Other

1. Explicitly add rubocop-performance, rubocop-rake, rubocop-rspec plugins
2. Remove unused Rails rules
3. Remove unused/deprecated RSpec/Layout/Style/Lint rules

[codeclimate]

Code Climate has analyzed commit 2ec263d and detected 0 issues on this pull request.

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