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

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

bank-account: add to track

hamming: update to v1.1.0 of canonical data

https://github.com/exercism/problem-specifications/blob/ebe9bbf952206384d344aded650407751a881e92/exercises/hamming/canonical-data.json

Updates now that two-fer is part of the track!

See
https://github.com/exercism/docs/blob/fe38450da526a35368e98fdb2f83ca40f1298aee/contributing-to-language-tracks/README.md#adding-hints