UX: add bulk-select to mobile topic lists

[awesomerobot]

This will make bulk-select topic-list actions available on mobile. We were previously hiding bulk select on mobile due to space constraints.

This adds a new nav button:

When the above nav button is clicked, avatars in the topic list are replaced by checkboxes, the bulk select bar sticks on scroll:

[CvX]

@awesomerobot I'm sorry I let this fall through!

Imo it makes sense to do it this way. 👍

In the future we can consider using something like a mix of Ember's in-element helper and our plugin outlets, so that a component could "donate" a button to e.g. d-navigation. Something like:

{{!-- topic-list.hbs --}}
{{#extend-component "d-navigation"}}
  {{d-button …something}}
{{/extend-component}}

…the rest of component

But for now this is completely fine. 😃

(to make up for the delay I rebased it to include latest main code and added a minor cleanup commit 😉)

[pmusaraj]

This is really nice. It looks good to me, only holding off approval because

• it's a slightly stale branch, we should rebase again and do a quick local test.
• it would be great if we could add a minimal test, would be the same as topic-bulk-actions-test but using the mobile layout.

Nice work @awesomerobot!

[SamSaffron]

This is a great change, any chance we can drive this to completion this week?

[awesomerobot]

@pmusaraj @SamSaffron should be good to go now!

[pmusaraj]

Looks great, just a few very minor comments. Thanks!

[discoursebot]

This pull request has been mentioned on Discourse Meta. There might be relevant details there:

https://meta.discourse.org/t/no-way-to-select-multiple-messages-for-bulk-action-on-mobile-view/69996/17