Self-assign issues on issue board (by hitting space)

Description

Now that we support multiple assignees, it should be easy to add yourself to issues when reviewing them on an issue board. Trello let's you add (and remove) yourself by hovering over a card and hitting space.

Proposal

Links / references

Documentation blurb

(Write the start of the documentation of this feature here, include:

1. Why should someone use it; what's the underlying problem.
2. What is the solution.
3. How does someone use this

During implementation, this can then be copied and used as a starter for the documentation.)