Add reference filter for design URLs

What does this MR do?

Adds support for parsing design URLs from markdown.

Fixes #217160 (closed)

Feature flags:

To enable design reference parsing, enable the design_management_reference_filter_gfm_pipeline feature flag:

[1] pry(main)> Feature.enable(:design_management_reference_filter_gfm_pipeline)
   (0.2ms)  BEGIN
  Feature::FlipperFeature Load (0.4ms)  SELECT "features".* FROM "features" WHERE "features"."key" = $1 ORDER BY "features"."id" ASC LIMIT $2  [["key", "design_management_reference_filter_gfm_pipeline"], ["LIMIT", 1]]
  Feature::FlipperFeature Create (0.5ms)  INSERT INTO "features" ("key", "created_at", "updated_at") VALUES ($1, $2, $3) RETURNING "id"  [["key", "design_management_reference_filter_gfm_pipeline"], ["created_at", "2020-05-18 22:46:06.056854"], ["updated_at", "2020-05-18 22:46:06.056854"]]
   (3.0ms)  COMMIT
   (0.2ms)  BEGIN
  Feature::FlipperGate Load (0.3ms)  SELECT "feature_gates".* FROM "feature_gates" WHERE "feature_gates"."feature_key" = $1 AND "feature_gates"."key" = $2  [["feature_key", "design_management_reference_filter_gfm_pipeline"], ["key", "boolean"]]
  Feature::FlipperGate Create (0.5ms)  INSERT INTO "feature_gates" ("feature_key", "key", "value", "created_at", "updated_at") VALUES ($1, $2, $3, $4, $5) RETURNING "id"  [["feature_key", "design_management_reference_filter_gfm_pipeline"], ["key", "boolean"], ["value", "true"], ["created_at", "2020-05-18 22:46:06.073131"], ["updated_at", "2020-05-18 22:46:06.073131"]]
   (1.4ms)  COMMIT
=> nil

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required) Documentation of feature flag needed
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Closes #217160 (closed)