Order issues / merge requests lists in both directions

Description

• This applies to these places: Project issues and project merge requests. Group issues and group merge requests.
• Your chosen sort and direction is saved for that particular type of list, and to your user. So that it remains even across devices. This is implemented in https://gitlab.com/gitlab-org/gitlab-ce/issues/50352 instead.
• When switching sorting type (e.g. Created date) we also switch the sorting direction (e.g. Descending) to that type's default direction. Example: Switching from Milestone to Created date sets the sorting direction to ↓ because it's the Created date default direction. Some sorting types don't allow you to switch direction (like Priority, see https://gitlab.com/gitlab-org/gitlab-ce/issues/39849#note_98128795).
• The default directions are https://gitlab.com/gitlab-org/gitlab-ce/issues/39849#note_98128795.
• Use this design with icons sort-highest and sort-lowest from gitlab-svgs:

sort-lowest / Low to high / ascending / A–Z / 1-9 / 1918–2018	sort-highest / High to low / descending / Z–A / 9-1 / 2018—1918

Original description

We use the weight to sort our issues and prioritise them for a while now. However, following the recent changes of the interface, the sorting dropdown only offers to sort the weight by lowest first, which is not useful at all as our issues with the lowest weight are the least important ones. We are really annoyed by this change and it is decreasing our efficiency so I thought I might as well create an issue here. I noticed that there was a discussion here #18651 (closed) about the length of the dropdown and thoughts about how to reduce it, and someone said something about a "flip" button but apparently it wasn't implemented. Is this going to change in the future?