Add banner to subscription action

[mazevedofs]

Phabricator: https://phabricator.wikimedia.org/T311639

Notes

• This task adds a banner to the subscription action
• We are not calling the API at this moment (but methods were added to the view model)
• The layout differs from figma, as we are using an existing component at the moment

Test Steps

1. Go to a talk page, and click the subscribe button. Make sure the alert shows at the bottom.
2. Click unsubscribe, verify the banner

[tonisevener]

Looking good! I didn't realize this view was so broken with bottom alignment and icons, thanks for working through it. I made a couple of minor tweaks in 966837e, hope that's okay:

1. There were some console constraint complaints, and after some digging it looks like there already was a leading constraint in the xib that your new titleViewLeading constraint conflicted with. So I exposed that xib constraint and am setting it to 50 instead.
2. I aligned some views to the superview safeAreaLayoutGuide instead of the extra safe area padding number. Both of these changes should only affect bottom and icon banners, so I think it's pretty safe.

[tonisevener]

Thanks for cleaning these up!