We forgot to do so in commit b896fc4bb. Back then, we said:
> Note that we aren't providing a proper aria-label for markers on the
> map we use in the form to create a proposal or an investment. Adding
> one isn't trivial given the current code, and keyboard users can't add
> a marker in the first place. We'll have to revisit this issue when we
> add keyboard support for this.
However, in the admin section, the marker is already there, so it should
have a label. In this case, we're using the coordinates as label because
it's the most relevant text for the marker in the context of a form. We
could also use "Default map location" instead, but that information is
already present on the page.
Axe was reporting the same accessibility error we mentioned in commit
b896fc4bb in this situation.
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.
