FR(SubmitPromise): dropdown list of politicians should be searchable 

**Is your feature request related to a problem? Please describe.**
As a Tracker, when I want to upload a new promise, I have to select the politician, the dropdown list is quite long and I have to scroll really far down if the politician's name is down in the list.

**Describe the solution you'd like**
I can key in the name and the list is shortlisted to names that match what I put in.
