feat: "Add member" button in org members list (#11848)

Fixes #1529.

This adds an "Add member" button to the list of members of an organization, offering a more intuitive way to add a user to an organization (instead of going through the list of teams).
This follows the design proposed in #1529. This PR can already be reviewed as such, but I plan to work on follow-up improvements:
- adding a confirmation dialog when adding the new member to the "Owners" team, since they get absolute rights on the org
- adding a text input to filter the list of teams, making it easier to select the desired teams when there are many of them
- potentially, improving the team creation link so that it brings the user back to the modal dialog once the team is created (but I'm not sure there's a ton of value behind this added complexity, since currently, creating a team will lead the user to the team page, which is a good place to add the member to the team)

This new way of adding members does not support issuing email invites, since we decided in #9884 that the invite feature hasn't got good enough of a UX to advertise it yet. Following [this discussion](https://codeberg.org/forgejo/discussions/issues/441), I am planning to work on enabling invites everywhere (potentially even making it the default).

## Checklist

### Tests for Go changes

- I added test coverage for Go changes...
  - [ ] in their respective `*_test.go` for unit tests.
  - [x] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I ran...
  - [x] `make pr-go` before pushing

### Tests for JavaScript changes

- I added test coverage for JavaScript changes...
  - [ ] in `web_src/js/*.test.js` if it can be unit tested.
  - [x] 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 plan to update https://docs.codeberg.org/collaborating/create-organization/#people once we are ready to take final screenshots of the feature.

### Release notes

- [x] This change will be noticed by a Forgejo user or admin (feature, bug fix, performance, etc.). I suggest to include a release note for this change.
- [ ] This change is not visible to a Forgejo user or admin (refactor, dependency upgrade, etc.). I think there is no need to add a release note for this change.

### Screenshots

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

## Release notes
<!--URL:https://codeberg.org/forgejo/forgejo-->
- Features
  - [PR](https://codeberg.org/forgejo/forgejo/pulls/11848): <!--number 11848 --><!--line 0 --><!--description IkFkZCBtZW1iZXIiIGJ1dHRvbiBpbiBvcmcgbWVtYmVycyBsaXN0-->"Add member" button in org members list<!--description-->
<!--end release-notes-assistant-->

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/11848
Reviewed-by: 0ko <0ko@noreply.codeberg.org>
Reviewed-by: Mathieu Fenniak <mfenniak@noreply.codeberg.org>
Co-authored-by: Antonin Delpeuch <antonin@delpeuch.eu>
Co-committed-by: Antonin Delpeuch <antonin@delpeuch.eu>

options/locale_next/locale_en-US.json

@@ -563,6 +563,10 @@
 	},
 	"teams.add_all_repos.modal.header": "Add all repositories",
 	"teams.remove_all_repos.modal.header": "Remove all repositories",
+	"members.add_member": "Add member",
+	"members.user": "User",
+	"members.user_already_member": "This user is already a member of the organization.",
+	"members.no_team_selected": "Organization members must belong to at least one team.",
 	"pulls.manual_merge.helper": "Manual merge helper",
 	"pulls.manual_merge.helpder.description": "Use this merge commit message when completing the merge manually.",
 	"pulls.manual_merge.commit.title": "Merge commit title",