Add links to liked and commented pages

Closes #5469

I still have some css and JS to write but it already works.
I think that these two text strings could be simply 'Likes' and 'Comments', or possibly 'My Likes' and 'My Comments' (for clarity and to match 'My Activity'), if people prefer those options. 'Liked posts' and 'Commented posts' feel a bit ugly to me, and also might suggest to some people that the view will be a stream of your own posts which other people have liked/commented on, so could create confusion.

Even if we retain 'Liked posts' and 'Commented posts', the second word in each should have an initial capital to match the other items in the list.