- Attach all HTTP handlers to a new `Handlers{}` struct.
- Remove all `handle*` function prefixes.
- Remove awkward, repetitive `app = c.Get("app").(*App)` from all handlers
and instead, simply access it from `h.app` from `Handlers{}`
Originally proposed in #2292.
- Make the beginning of handlers consistent with uniform variable declaration
and grouping.
- Add missing comments.
- Fix staticcheck/vet warnings and idiom issues.
- Move user models from `/models` to `internal/auth`.
- Move and refactor various permission check functions into `User.()`
- Refactor awkward `get, manage bool` function args into `Get|Manage` bitflags.
- Don't setup username+password from config file during fresh install.
- If `LISTMONK_ADMIN_USER` and `LISTMONK_ADMIN_PASSWORD` env vars are
set during `--install`, use them.
- Otherwise, render new username+password creation UI on `/admin/login`.
- Add Cypress tests.
