Add slash command for weight

There currently doesn't appear to be a slash command that allows you to change the weight of an issue. While we don't use weight often at GitLab, for teams that use it, I expect a slash command would be quite useful.

I expect the options would be like:
/weight <none | 1 | 2 | 3 | 4 | 5 | 6 | 7>

If this gets added, we should also remember to update https://gitlab.com/help/user/project/slash_commands