INT-4453: Move SI Twitter module to Extensions

[artembilan]

JIRA: https://jira.spring.io/browse/INT-4453

Since Spring Social project is aiming to its EOL, there is no reason to keep such a dependency in the Core project

• Create a spring-integration-social-twitter project
• Move all the sources and tests from the SI Core to this project
• Rename spring-integration-twitter-5.1.xsd to spring-integration-social-twitter-1.0.xsd do not confuse IDE for auto imports of XSDs
• Move twitter.adoc to README of this extension project
• Replace RedisMetadaStore in test-case to a SimpleMetadataStore since this integration test does nothing with Redis and we don't have access to the @RedisAvailable JUnit rule
• Enable all the Checkstyle rules and fix violations

[garyrussell]

I guess we'll need a bamboo build plan.

[artembilan]

I have added the CI plan: https://build.spring.io/browse/INTEXT-SIST, but it looks like something is still missed:

21-Jul-2018 00:28:26 | Exception executing command "./gradlew clean build  dist --no-daemon -d -I  /tmp/artifactory.init.script6512705478035841731gradle  -DbuildInfoConfig.propertiesFile=/tmp/buildinfo5839690595938776439properties  artifactoryPublish -Dartifactory.resolve.password=********  -Dartifactory.publish.password=******** ./gradlew could not be  startedcom.atlassian.utils.process.ProcessNotStartedException: ./gradlew  could not be started
-- | --
21-Jul-2018 00:28:26 | Failing task since return code of [./gradlew clean  build dist --no-daemon -d -I  /tmp/artifactory.init.script6512705478035841731gradle  -DbuildInfoConfig.propertiesFile=/tmp/buildinfo5839690595938776439properties  artifactoryPublish -Dartifactory.resolve.password=********  -Dartifactory.publish.password=********] was -1 while expected 0

My best guess that gradlew is not executable in the project. My standard Windows fault 😢