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.
* I namespaced `Symbol` into the module `OntologyMember` as suggested by @0robustus1 because the class `Symbol` is already taken by ruby.
* I manually adjusted the generated migration because it is not aware of naming conventions in join tables. See b8559df and 93d7abd for details.
* The migration requires PostgreSQL in version 9.2 or greater because we rename constraints.

What I think needs to be done yet:
- [ ] Check everything on graphs, i.e. revert the renaming in the following files, where needed:
  * 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 the functions in the new migration
  - [ ] Make graphs work (e.g. http://ontohub.dev/repositories/default/ontologies/5/graphs) where 5 is the id of `strict_partial_order`
- [x] Check the renaming in all the other files ;)
- [ ] Check if `lib/triple_store.rb` actually needs renaming. I think it doesn't.
- [x] Check if everything is correct in `config/locales/en.yml`
- [x] Check if there are still some entities and links that should be renamed.
- [x] Test the rendering of `app/views/mappings/*.html.haml`
- [x] Test the rendering of `app/views/symbols/*.html.haml`
- [x] Check if new routes are used
  - [x] links/mappings
  - [x] entities/symbols
- [ ] Have green tests
- [ ] Check if the migrations are correct on develop.ontohub.org
- [ ] Obey Hound. I'd like this to be the last step before the merge because the diff would get confusing otherwise.
- [ ] Have 4 thumbs up
Style/StringLiterals: Prefer single-quoted strings when you don't need string interpolation or special symbols.