circular-buffer: add to track

Update to Gradle 4.0.1

https://twitter.com/gradle/status/883366530980864005

Update policy around language constructs

list-ops: update to match Kotlin track

Removed starter implementations for exercises 21 and beyond, according to POLICIES.md.

bowling: add to track

poker: sort settings.gradle and tidy up

Fix formatting and Intellij warnings. Remove starter implementation as
this is after the first 20 exercises. Move poker in settings.gradle so
that it's sorted alphabetically.

sieve: create and validate starter implementation

twelve-days: create and validate starter implementation

Move contributing info into CONTRIBUTING file and add mention of rebases

spiral-matrix: add to track

Based on v1.0.0 of the canonical data:

https://github.com/exercism/problem-specifications/tree/28a32203b96ebaecac6a6968831bd66718d8ba30/exercises/list-ops

Plus modifications that are being discussed in

https://github.com/exercism/problem-specifications/issues/826

complex-numbers: add to track

Standardize order of test annotations

Create RESOURCES.md and add resources

Clean up exception-related hints

Formatted Stack Overflow with a space in between.
Capitalized Java in the Reddit link and removed the claim that it was official.
Removed Gitter room link
The documentation is official

journey test: fix for trackler submodule rename

https://github.com/exercism/trackler/commit/69e055ad52b3f8d19da95b6ab0593678b8ee1bb4

list-ops: estimate difficulty 9

poker : add to track