block_timeline: Filter icon in sort dropdown is misleading

In the timeline block, there is a dropdown with a filter icon which can be used to sort the list. As the same icon is also used in block_myoverview to filter the course list, this filter icon is misleading in this context.

I would propose to replace this .fa-filter icon with .fa-long-arrow-down.

Additionally, in the dropdown, the entry "Sort by dates" could be prefixed with .fa-sort-numeric-asc and the entry "Sort by courses" could be prefixed by .fa-sort-alpha-asc if this makes sense to you.