github: Add simple issue templates & configuration

[neiljp]

What does this PR do, and why?

This converts the basic issue filing system to use the GitHub template chooser.

The main feature at this point is to focus reporters on the scope of the issue:

• major bug (bug: crash)
• bug (bug)
• missing Zulip feature (missing feature)
• general feature/improvement (enhancement)

These should apply a corresponding default label to the issue, with the idea that this would provide an initial structure to fine-tune the labels, eg. a specific parity label to add to missing feature, which could also gain one of the user or admin sub-labels.

This already feels like an improvement over the current single blank 'template', though it may take time to see how it works in practice.

This takes some ideas from zulip/zulip#26000 and followups.

Outstanding aspect(s)

• How do these categories look like, if you were filing an issue?

External discussion & connections

• [x] Discussed in #zulip-terminal in Issue types #T1488
• [ ] Fully fixes #
• [x] Partially fixes issue #830
• [ ] 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

• [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
• [x] 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

[neiljp]

@Niloth-p Does the fixup commit address your suggestions?

[neiljp]

Pushed a few more minor changes. Will look at this again later, but I'd like to squash and merge this to get some real-life testing going, so we can iterate further if necessary.