This PR is the first step towards replacing our `{{user-selector}}` and eventually deprecating and removing it from our codebase. Some of `{{user-selector}}` problems are:
1. It's called `{{user-selector}}`, but in reality in can also select groups and emails.
2. It's an Ember component, yet it doesn't have a handlebars template and uses jQuery to render itself and modify the DOM. An example of this problem is when you want to clear the selected users programmatically, see [this](6c155dba77/app/assets/javascripts/discourse/app/components/user-selector.js (L179-L185)).
3. We now have select kit which does very similar things but a lot better.
This PR introduces `{{email-group-user-chooser}}` which is meant to replace `{{user-selector}}`. It extends select kit and has the same features that `{{user-selector}}` has. `{{user-selector}}` is still used in a few places in core, but they'll all be replaced with the new component in a separate commit.
Once `{{user-selector}}` is not used anywhere in core, it'll be deprecated and then removed after the 2.7 release.
Installing multiple copies of the same theme/component is possible, but you rarely need to actually have multiple copies installed. We've seen many times new admins installing duplicates of components because they were unaware it was already installed. This PR makes the theme installer modal loop through the existing themes when you click on 'install', and if there is a theme with a URL that matches the URL you entered, a warning will show up and you will need to click 'install' again to proceed.
Users could be silenced or suspended by two staff members at the same time and
would not be aware of it. This commit shows an error message if another penalty
has been applied.
