Add member autocomplete to mission runners field on mission edit page

[ZMYaro]

Currently, the mission runners field is a plain text field that takes comma-separated member UUIDs. It should be replaced with a field that autocompletes member names and creates the list of UUIDs under the hood.

[ZMYaro]

Example of what it could look like:

[ZMYaro]

This will be assigned to @jeanpratt once she joins the GitHub project.

[ZMYaro]

We decided to go with autocomplete.js unless a reason comes up to reconsider.

[ZMYaro]

Pushing back to R13 as discussed at 2018-04-19 meeting.