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