Ide render whitespaces

What does this MR do?

Render whitespaces in Web IDE

Replaces gitlab-foss!24862 (closed).

Screenshots

Description	Screenshot (90a28dc4 on 2019-11-12)
User preferences
When enabled, IDE shows whitespace as faded dot
When disabled, IDE is the same

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Remaining

• Add screenshots to this MR (of the user preferences and editor).
• UX review
• 🔨 Add FE unit tests
• 🔨 Add BE unit tests
• 🔨 Add feature spec for new user preference field
• 🔨 Add feature spec for IDE when render whitespace is enabled / disabled (there might be issues with this)... It looks like there are issues with feature specs in the IDE, so we can just assert that the data attribute gets set correctly.
• FE review
• BE review
• Merge 🚀