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.

• [x] The tests are failing for reactions_view

Discussed in zulip-terminal > reaction metadata

External discussion & connections

• [x] 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?

• [x] Manually - Behavioral changes
• [x] Manually - Visual changes
• [x] Adapting existing automated tests
• [x] 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

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