The old choose-topic component did not have the same style as the rest
of the create invite modal and was not very suitable to use in the modal
because it introduced the search results in modal's body.
The new topic-chooser is built using select-kit and provides a more
polished user experience.
User flair was given by user's primary group. This PR separates the
two, adds a new field to the user model for flair group ID and users
can select their flair from user preferences now.
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.
