After adding it as a separate field, I figured out that we should use
the same picker, because otherwise we'd have to keep the date in sync
depending on the time zone. Zones are too messy in that regard, and the
date is already normalized to UTC before creating the contribution
record.
closes#141
* Adds a new page for updating profiles
* Refactors the add-contributor component to allow for updating
existing contributors
* Adds input labels to the contributor form and improves placeholders
* Adds event handlers for all contract contributor changes and uses them
for updating the UI
refs #122
basti