messages: Update metadata handling for message reaction.

[mounilKshah]

What does this PR do, and why?

Reaction metadata has been changed in the messages API endpoint as per ZFL 2 in Zulip 3.0. The user_id is used to identify the reaction's user.

• The tests are failing for reactions_view

Discussed in zulip-terminal > reaction metadata

External discussion & connections

• Discussed in #zulip-terminal in reaction metadata
• Fully fixes #
• Partially fixes issue #
• Builds upon previous unmerged work in PR #
• Is a follow-up to work in PR #
• Requires merge of PR #
• Merge will enable work on #

How did you test this?

• Manually - Behavioral changes
• Manually - Visual changes
• Adapting existing automated tests
• Adding automated tests for new behavior (or missing tests)
• Existing automated tests should already cover this (only a refactor of tested code)

Self-review checklist for each commit

• It is a minimal coherent idea
• It has a commit summary following the documented style (title & body)
• It has a commit summary describing the motivation and reasoning for the change
• It individually passes linting and tests
• It contains test additions for any new behavior
• It flows clearly from a previous branch commit, and/or prepares for the next commit