feat: Global 2FA enforcement (#8753)

resolves #8549

This PR add a config to enforce 2FA for the whole Forgejo instance. It can be configured to `none`, `admin` or `all`.
A user who is required to enable 2FA is like a disabled user. He can only see the `/user/settings/security`-Page to enable 2FA, this should be similar to a user which needs to change his password. Also api and git-commands are not allowed.

## Checklist

The [contributor guide](https://forgejo.org/docs/next/contributor/) contains information that will be helpful to first time contributors. There also are a few [conditions for merging Pull Requests in Forgejo repositories](https://codeberg.org/forgejo/governance/src/branch/main/PullRequestsAgreement.md). You are also welcome to join the [Forgejo development chatroom](https://matrix.to/#/#forgejo-development:matrix.org).

### Tests

- I added test coverage for Go changes...
  - [x] in their respective `*_test.go` for unit tests.
  - [x] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I added test coverage for JavaScript changes...
  - [ ] in `web_src/js/*.test.js` if it can be unit tested.
  - [ ] in `tests/e2e/*.test.e2e.js` if it requires interactions with a live Forgejo server (see also the [developer guide for JavaScript testing](https://codeberg.org/forgejo/forgejo/src/branch/forgejo/tests/e2e/README.md#end-to-end-tests)).

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [ ] I did not document these changes and I do not expect someone else to do it.

I will do it, if the general idea of this PR is a good feature.

### Release notes

- [ ] I do not want this change to show in the release notes.
- [x] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.

<!--start release-notes-assistant-->

## Release notes
<!--URL:https://codeberg.org/forgejo/forgejo-->
- Security features
  - [PR](https://codeberg.org/forgejo/forgejo/pulls/8753): <!--number 8753 --><!--line 0 --><!--description R2xvYmFsIDJGQSBlbmZvcmNlbWVudA==-->Global 2FA enforcement<!--description-->
<!--end release-notes-assistant-->

Co-authored-by: 0ko <0ko@noreply.codeberg.org>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8753
Reviewed-by: 0ko <0ko@noreply.codeberg.org>
Reviewed-by: Ellen Εμιλία Άννα Zscheile <fogti@noreply.codeberg.org>
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: zokki <zokki.softwareschmiede@gmail.com>
Co-committed-by: zokki <zokki.softwareschmiede@gmail.com>

templates/base/head_navbar.tmpl

@@ -24,7 +24,7 @@
 		</div>
 
 		<!-- navbar links non-mobile -->
-		{{if and .IsSigned .MustChangePassword}}
+		{{if and .IsSigned .HideNavbarLinks}}
 			{{/* No links */}}
 		{{else if .IsSigned}}
 			{{if not .UnitIssuesGlobalDisabled}}
@@ -54,7 +54,7 @@
 
 	<!-- the full dropdown menus -->
 	<div class="navbar-right ui secondary menu">
-		{{if and .IsSigned .MustChangePassword}}
+		{{if and .IsSigned .HideNavbarLinks}}
 			<div class="ui dropdown jump item" data-tooltip-content="{{ctx.Locale.Tr "user_profile_and_more"}}">
 				<span class="text tw-flex tw-items-center">
 					{{ctx.AvatarUtils.Avatar .SignedUser 24 "tw-mr-1"}}