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.
The `use_helpers` method was added in ViewComponent 3.8.0, and it's
included by default in all components since version 3.11.0.
Note we sometimes delegated the `can?` method to the controller instead
of the helpers, for no particularly reason. We're unifying that code as
well.
Since now categories are loaded in both the investment form component
and proposal form component, and their controllers are the only
"commentable" controllers using the `@categories` instance variable, we
can remove the `load_categories` call in `CommentableActions` affecting
the create and update actions.
