Make dashboard generation project-agnostic

@jcommelin raised the idea of making the dashboard generation to other projects, i.e. allowing other projects (on github, but perhaps also on other platforms) to re-use the same logic. In fact, the frontend of this project ("take a bunch of PR metadata and generate the dashboard webpage(s) from it") is pretty agnostic --- so this could be done!

The following is the current state of the various parts of the project front-end.

• util.py and ci_status.py are fully generic; nothing to do
• classify_pr_state.py contains the particular classification logic (which is somewhat mathlib-specific); perhaps this can be made configurable (see below)
• state_evolution.py has a mild mathlib dependency: it hard-codes the organisation name, and the "on_the_queue" methods assume there's a PRStatus variant AwaitingReview
• mathlib_dashboards.py is entirely mathlib-specific; could be replaced by a configuration file in the future
• compute_dashboard_prs.py: first half is mathlib-agnostic (100 lines); second half is mathlib-specific
• dashboard.py: first half is mathlib-agnostic (400 lines); webpage layout is project-specific for now
• generate_assignment_page.py is considered internal-only for now (but should not be hard to generalise if needed)
• send_zulip_dm.py: completely project-agnostic; currently unused, but could be used in generate_assignment_page.py

(The files process.py and check-data-integrity.py are part of the backend, hence not relevant.)

Review util.py, ci_status.py, classify_pr_state.py, state_evolution.py, mathlib_dashboards.py, compute_dashboard_prs.py, and dashboard.py to map the current generic and mathlib-specific boundaries. The issue does not define a specific implementation or acceptance test; completion would require an agreed configuration or architecture that lets other projects reuse dashboard generation.