Add multi-category support for key bindings.

[Niloth-p]

What does this PR do, and why?

Break the restriction that a hotkey can only belong to a single category, allowing a hotkey to be displayed in multiple relevant sections of the help menu.

Outstanding aspect(s)

This does not currently use the new infrastructure to update particular key bindings assigning them to multiple categories.
Because they depend on the newer categories that are being created in prior PRs.

Further discussion required on the amount of duplication that would be acceptable in the help menu, and whether to only include entries in their type-based categories, or in both type-based as well as context-based categories.

External discussion & connections

• Discussed in #zulip-terminal in topic
• 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

[zulipbot]

Hello @zulip/server-hotkeys members, this pull request was labeled with the "area: hotkeys" label, so you may want to check it out!