Adds configurable topic settings via yaml block

Adds a feature that allows the discoto summary to be configurable on a per-topic basis. Default settings are:

summary:
  max_items: -1              # An integer. Values less than zero mean no limit
  sort_by: created           # One of created, updated
  sort_direction: ascending  # One of ascending, descending

Checklist

• Manually verified passing unit tests for this branch locally by running bin/run_docker_tests.sh
  • Yes, this is a pain, but GitLab doesn't run reasonably within a docker container in a normal CI pipeline
  • See bin/run_docker_tests.sh --help for faster/iterative testing options
• Paste local test output
• Add a CHANGELOG.md entry

Local Test Output

/bin/run_docker_tests.sh output

 lptp [ discussion-automation ]: bin/run_docker_tests.sh --no-kill
[>>>] Will not kill docker container (can be re-used)
[>>>] Retrieving token
[>>>] GitLab should be ready
[>>>] Pasteable config into bin/.env: 
[>>>] 
export DISCOTO_GITLAB_ENDPOINT=http://172.17.0.2
export DISCOTO_GITLAB_TOKEN=J7oR-BpcL48SFmKp26ga
export DISCOTO_PROJECTS=test-group/test-project
export DISCOTO_GROUPS=test-group
export DISCOTO_ISSUABLE_ASSIGNEE=
export DISCOTO_ISSUABLE_AUTHOR=
export DISCOTO_ISSUABLE_LABELS=discoto
[>>>] 
[>>>] Running tests on 23-configurable_discoto @ 500ad7956e3d3b1d8add5ef1e805bc365298218f
warning: parser/current is loading parser/ruby27, which recognizes
warning: 2.7.1-compliant syntax, but you are running 2.7.2.
warning: please see https://github.com/whitequark/parser#compatibility-with-ruby-mri.
.................................W, [2021-03-24T14:16:14.660962 #1176049]  WARN -- : Discoto::SummarySettings: Data validation errors in summary settings YAML
W, [2021-03-24T14:16:14.660993 #1176049]  WARN -- : Discoto::SummarySettings:   The property '#/max_items' of type string did not match the following type: integer in schema 18afbda5-bcc4-523e-9131-ee2211df885d
W, [2021-03-24T14:16:14.661005 #1176049]  WARN -- : Discoto::SummarySettings:   The property '#/sort_by' of type integer did not match the following type: string in schema 18afbda5-bcc4-523e-9131-ee2211df885d
W, [2021-03-24T14:16:14.661014 #1176049]  WARN -- : Discoto::SummarySettings:   The property '#/sort_by' value 1 did not match one of the following values: created, updated in schema 18afbda5-bcc4-523e-9131-ee2211df885d
W, [2021-03-24T14:16:14.661023 #1176049]  WARN -- : Discoto::SummarySettings:   The property '#/sort_direction' of type object did not match the following type: string in schema 18afbda5-bcc4-523e-9131-ee2211df885d
W, [2021-03-24T14:16:14.661032 #1176049]  WARN -- : Discoto::SummarySettings:   The property '#/sort_direction' value {"hello"=>10} did not match one of the following values: ascending, descending in schema 18afbda5-bcc4-523e-9131-ee2211df885d
.W, [2021-03-24T14:16:14.661399 #1176049]  WARN -- : Discoto::SummarySettings: Data validation errors in summary settings YAML
W, [2021-03-24T14:16:14.661417 #1176049]  WARN -- : Discoto::SummarySettings:   The property '#/' of type null did not match the following type: object in schema 18afbda5-bcc4-523e-9131-ee2211df885d
........................

Finished in 2 minutes 34.3 seconds (files took 0.44532 seconds to load)
58 examples, 0 failures

Coverage report generated for RSpec to /home/james/ws/dev/gitlab/discussion-automation/coverage. 728 / 828 LOC (87.92%) covered.
[>>>] Cleaning up
[>>>] Cleaning up

Closes #23 (closed)