TOPICS: 'Group issue' - logged out UX for engaging in public topics

[BeccaMelhuish]

Making an issue to group together these issues which are all related to the UX of trying to engage in a public topic when logged out:

• IDEA GATHERING: When not logged in, clicking 'reply' leads button to disappear #1636
• DISCUSSIONS: When not logged in, argument 'reply' buttons are missing #1637
• MISC: When logged out and trying to participate, make it clearer why login pop-up appears.  #1640
• IDEA GATHERING: When not logged in, 'like idea' button is greyed out, should be active but trigger login pop-up #1639

These should all be fixed before the 'view without joining topic' button in #1701 is implemented.