Create models for Issue trackers (properties) fields

We need to extract these fields from the JSON-serialized properties field so that we can properly encrypt the sensitive fields.

The first iteration is to create needed models.

Issue tracker services that we currently support:

• Bugzilla
• Custom Issue Tracker
• Redmine
• YouTrack
• Jira

The fields they use are similar except for Jira

Some things to note:

• URL fields may also be sensitive since it may contain tokens / keys
• Since we don't have an alternative to attr_encrypted yet, we can continue to use that