* GH-3438: Add Build from Source into CONTRIBUTING
Fixes https://github.com/spring-projects/spring-integration/issues/3438
It is not clear at the moment what Java version and what build system is used
in the project from the `CONTRIBUTING.adoc`
* Add `Build from Source` section into the `CONTRIBUTING.adoc`
* Point to the `CONTRIBUTING.adoc` from the top level `README.md`
* Fix the current Java version in the `README.md`
* Mention that `Squash commits` must be avoided for subsequent commits into the PR
* * Apply suggest change for docs result
* Change Travis config to use `oraclejdk11` which is minimal already for for the project
* Remove `CODE_OF_CONDUCT.adoc` if favor of the same for a GitHub org
* Move `LICENSE.txt` to the root project dir
* Fix `build.gradle` to copy a `LICENSE.txt` from the actual location
* Fix `README.md` and `CONTRIBUTING.adoc` to point into the actual `CODE_OF_CONDUCT`
& Security policy
* Add `ISSUE_TEMPLATE` directory with files similar to ones in `spring-session`
* Fix wrong section `id` in the `whats-new.adoc`
* Do not fix urls for the `http://apache.org/xml/features` when we
configure XML components
* Do not fix urls for localhost and fake links in tests
* Fix all the dead links in docs
