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>

templates/admin/user/view.tmpl

@@ -6,7 +6,9 @@
 			<h4 class="ui top attached header flex-left-right">
 				<span>{{.Title}}</span>
 				<span class="flex-text-block">
-					<button type="button" class="ui primary compact tiny basic button link-action" data-url="{{.Link}}/impersonate">{{ctx.Locale.Tr "admin.users.impersonate"}}</button>
+					{{if not .User.IsTypeBot}}
+						<button type="button" class="ui primary compact tiny basic button link-action" data-url="{{.Link}}/impersonate">{{ctx.Locale.Tr "admin.users.impersonate"}}</button>
+					{{end}}
 					<a class="ui primary compact tiny button" href="{{.Link}}/edit">{{ctx.Locale.Tr "admin.users.edit"}}</a>
 				</span>
 			</h4>
@@ -23,6 +25,9 @@
 			</div>
 		</div>
 	</div>
+	{{if .BotAccessTokens}}
+		{{template "shared/user/access_tokens" .BotAccessTokens}}
+	{{end}}
 	<h4 class="ui top attached header">
 		{{ctx.Locale.Tr "admin.repositories"}} ({{ctx.Locale.Tr "admin.total" .ReposTotal}})
 	</h4>