Implement LiquidCachedAssigns

[aapomm]

Spec

The LiquidAssignsService does a lookahead of the content but this can be skipped if we implement a lazily loaded liquid assigns that would simplify the code.

Proposed solution
To lazily load the drops, we implement a LiquidCachedAssigns class to replace the LiquidAssignsService and does the following:

• quacks like a hash {}
• when it receives a message, it validates if it's associated with a list of records ('issues', 'notes', etc.)
  • if valid, it fetches the records from the project
  • if invalid, check if if it present in the current @assigns hash (to catch the case for record-based assigns)

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Check List

• Added a CHANGELOG entry