Add dealing with newlines for editor to 5.1-to-6.0

backport of https://github.com/gitlabhq/gitlabhq/commit/2d681c0d1ed14bf23c041aa04fa3e77caceda9a1
Fix env varable style for documentation
Merge pull request #7537 from Razer6/rake_tasks_style

Fix env variable style for documentation
Merge pull request #7438 from bbodenmiller/patch-5

Add dealing with newlines for editor to 5.1-to-6.0
Add 7.1-to-7.2.md update guide
Add 6.0-to-7.2.md guide ahead of 7.2 release
Test if the label is valid before creating link
Remove invalid characters from the label name
Use the default color when migrating labels
Merge branch 'fix_label_migration' into 'master'

Fix label migration

Failure to create any labels at all caused this DB migration to run for over 40
minutes during a gitlab.com deploy this morning. This change specifies a color f
or each new label and strips invalid characters from the label name.

See merge request !1024
Bump version to 7.2.0.rc2
Improve labels validation and expose error messages
Add spec on labels validation