Prepare 2.4

This pull request prepares Symphony 2.4. It fixes or includes the following issues:

- [x] pull request #1887: Improve UI performance
- [x] pull request #1587: Add datasource negation for SectionDatasource and AuthorDatasource, including my UI changes available in https://github.com/nilshoerrmann/symphony-2/tree/datasource-negation-2.4
- [x] #1663 / #1888: Relax our CSS styling
- [x] #1873 / #1890: Dynamically fetch event documentation (part 1)
- [x] #1823 / pull request: #1891: Current-query-string parameter should not contain '?' 
- [x] #1890: Dynamically fetch event documentation (part 2)
- [x] pull request #1547: Changeable admin url _(needs to be rebased)_
- [x] pull request #1870: Made the selectable input selector more precise _(needs some clean-up)_

There are a few breaking changes that need to be documented in the release notes:

- Changes to the JavaScript and CSS API, see #1887 
- Changes to the query string parameter, see #1891