Few fixes and features

This pull request fixes a few things. I suppose styles could be broken since I changed the views a bit to reduce code redundancy.

I believe I fixed issues #561, #443, #592 (already marked closed though) and #569

Besides that, a few unreported issues were fixed by commits
- 1145333536ea58fe031e19cfbbe8473b1e123045 (forum.last_visible_post returned hidden posts)
- cd39afef374ddb45bd15ec9cbc9f7a0c95dece0a (show member post count and member sice)

It's my first contribution to a rails engine, so I'm pretty sure I'm not doing it the right way. Please let me know what needs to change to allow my code to be pulled in.
But since you're asking, what it does is it hides the topics count columns on small screens so the forum overview is somewhat functional. Try https://www.lab-nation.com/forum and resize your browser window really narrow.