Add manual cycling of random hints in footer

[Niloth-p]

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What does this PR do, and why?

Enables cycling through random hints in the footer.

• Using a hotkey to manually go to next hint
• Periodically cycle through footer hints automatically

This is in preparation to #1484, where we add contexts and the footer displays only the context-specific hotkey hints.

Outstanding aspect(s)

• If this is good, the next step could involve making the footer's cycle period a user setting.
• Deciding the hotkey to go to next hint: I've currently used Tab, but that's subject to change on discussion.
• Deciding the default cycle period: I've currently set it to 30 seconds, but we could increase it.

External discussion & connections

• [x] Discussed in #zulip-terminal in Adding context to hotkeys, Cycling through footer hints, T1484's review
• [ ] 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
• [ ] Manually - Visual changes
• [ ] 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)
• [ ] It has a commit summary describing the motivation and reasoning for the change
• [x] It individually passes linting and tests
• [x] It contains test additions for any new behavior
• [x] 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!

[neiljp]

I left some notes in the channel, but getting the first commit merged with an adjusted key would be great as a first step. The remainder could be useful, particularly since refactoring to make it clearer to understand could benefit if we keep the timed approach, but could also depend on the style of help hints we settle on when they're more contextual.

[neiljp]

@Niloth-p Thanks for the quick PR adjust, which I only just noticed. The only change I made was to make it clearer that the 'hint' being referred to is an regard to a hotkey :)