cleanup - make pom.xml files more consistent

fix Javadoc

This helps with the situation where multiple Jenkins instances exist in memory (at the same time or at different times sequentially), such as during unit tests.

RunIdMigrator.LEGACY_ID_FORMATTER was static, which gets instantiated before the time zone is overriden during the test. Changing this to an instance field so that it gets created later on.

[FIXED JENKINS-25897] Add range check for H(X-Y) syntax

- Recorded the fix in changelog.
- Added a separate screen to show the unmigration instruction, which
  makes it easier for people to find it than search logs.

This should work across builds and even across jobs, and does not force us to load other builds.

Add pom.xml.versionsBackup and .DS_Store to .gitignore

Making side-panel and main-panel absolute positioned and full height