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