allow limited HTML tags from group blog posts to be displayed in activity items  

Currently, simple HTML tags in group blog post titles, like `News from the <em>MLA Commons</em>`, are rendered literally when they're displayed as activity items (see mlaa/cbox-mla#128). This seems to be because bp-groupblog is sanitizing them on the way out. This fix uses `wp_kses` instead of `esc_attr()`, to filter out all tags except for `<em>` and `<strong>`, allowing us to apply simple formatting to post titles and have them displayed correctly in their resulting activity items.