This way we remove duplication.
Note that to check whether to render the button to remove a marker,
we're checking whether the map location belongs to a mappable. This
means we're changing the code that renders the map in the "new proposal"
and "new investment" forms so the map location belongs to a proposal or
investment. We're association the map location to a new record because
writing something like:
```
def map_location
proposal.map_location || MapLocation.new(proposal: proposal)
end
```
Would change the `proposal` object because of the way Rails treats
non-persisted `has_one` associations. Although probably safe in this
case, changing an object when rendering a view could have side effects.
Also note that we're changing the HTML ID of the map element from
`admin-map` to `new_map_location` (the latter is returned by the
`dom_id` method). We were only using this ID in tests since commit
289426c1c, so changing it doesn't really affect us.
