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>

modules/structs/admin_user.go

@@ -77,3 +77,12 @@ type EditUserOption struct {
 	// User visibility level: public, limited, or private
 	Visibility VisibilityString `json:"visibility" binding:"In(,public,limited,private)"`
 }
+
+// ConvertUserTypeOption options when converting a user between individual and bot
+type ConvertUserTypeOption struct {
+	// The target user type
+	//
+	// required: true
+	// enum: ["User","Bot"]
+	UserType UserTypeString `json:"user_type" binding:"Required;In(User,Bot)"`
+}