Ability to post beatmap comments on beatmap discussion page

[minetoblend]

Depends on #74 and ppy/osu-web#10199

Users should be able to choose to have their in-editor comments posted on the osu website.
This should be opt-in to avoid posting stuff on the user's behalf without them wanting that. If the comment has a drawing, etc. attached it should post either a screenshot alongside it, or add a link to view it in osucad.

Since there's no guarantee that osu's api will ever allow creating beatmap discussion posts, it's uncertain if adding this feature will be possible.