feat: manage bot accounts from the admin UI, API and CLI (#38966)

Adds first-class bot accounts (`UserTypeBot`): local, password-less
users for automation that authenticate only with access tokens.

1. Admin UI: create bots, filter users by type, manage a bot's access
tokens, convert between user and bot
2. API: `POST /admin/users/{username}/convert-type`, and user objects
gain a GitHub-compatible `type` (`User`, `Organization`, `Bot`)
3. CLI: `gitea admin user change-type`, `--user-type` accepts `User` or
`Bot` case-insensitively
4. Converting keeps the password, 2FA, OAuth2 grants and access tokens,
and since sign-in rejects bots, converting back restores the account.
Only local, non-admin accounts can be converted, and conversions are
audited
5. Session, reverse proxy, SSPI, external source and password reset
sign-in reject non-individual users, so a bot never gets an interactive
session
6. Bots receive no notifications or emails

Co-authored-by: Nicolas <bircni@icloud.com>
Co-authored-by: joestump <joe@joestump.net>
Co-authored-by: Joe Stump <joe@stu.mp>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

models/user/error.go

@@ -86,3 +86,9 @@ func IsErrUserIsNotLocal(err error) bool {
 	_, ok := err.(ErrUserIsNotLocal)
 	return ok
 }
+
+var (
+	ErrBotCanNotBeAdmin      = util.NewInvalidArgumentErrorf("bot user can not be a site administrator")
+	ErrBotMustBeLocal        = util.NewInvalidArgumentErrorf("bot user must use local authentication")
+	ErrUserTypeCanNotConvert = util.NewInvalidArgumentErrorf("user type can not be converted")
+)