455 finally changing the model names

Here, the renaming script of #455 is already executed. I already adjusted many things, but there are still a lot of manual adjustments needed.

To recall: There are two models that are supposed to be renamed:
* `Entity` wants to be known as `Symbol`
* `Link` wants to be called `Mapping`. This one makes the diff really hard to understand because there are several kinds of links and we only want to rename what is related to the model.

What might be important to know while reading the diff:
* `:unprocessable_entity` was renamed. I'm pretty sure, this meant the HTTP status code 422.
* I renamed staging's `MappingsController` to `GeneralMappingsController` because there was a `LinksController` which would have overwritten the `MappingsController`. This means that also the views have been moved to `app/views/general_mappings/`.
* There already was a `spec/factories/mappings.rb` for the subclasses of `GeneralMappingsController` and a `spec/factories/links.rb`. I merged those factories into a single file.
* In `app/models/ontology/mappings.rb`, there we have a `linkid`. This needs to stay as is, because it comes from the xml files hets yields.

What I think needs to be done yet:
- [ ] Check everything on graphs, i.e. revert (most of) the renaming in the following files:
  * app/assets/javascripts/plugins/graph.js.coffee
  * app/assets/stylesheets/graphs.css
  * app/controllers/graphs_controller.rb
  * app/models/ontology.rb (`include GraphStructures::SpecificFetchers::Mappings`)
  * app/views/graphs/index.html.haml
  * lib/graph_data_fetcher.rb
  * lib/graph_structures/specific_fetchers/mappings.rb
  * spec/models/graph_data_fetcher_spec.rb
- [ ] Check if `lib/triple_store.rb` actually needs renaming. I think it doesn't.
- [ ] Check if everything is correct in `config/locales/en.yml`
- [ ] Check if there are still some entities and links that should be renamed.
- [ ] Test the rendering of `app/views/mappings/*.html.haml`
- [ ] Test the rendering of `app/views/symbols/*.html.haml`
- [ ] Have green tests
- [ ] Have 4 thumbs up
Style/IfUnlessModifier: Favor modifier `if` usage when having a single-line body. Another good alternative is the usage of control flow `&&`/`||`.