authors is no longer a valid keyword.
In this commit the corresponding functionality
is removed. It will be replaced in the future.

keywords was renamed to keyword.
Change some variable names accordingly.
(Some changes might be unnecessary and could be reverted.
I prefer continuing the usage of keywords,
since it is a more logical name for a list of keywords.)

programming_languages was renamed to
programmingLanguage.
Most corresponding variable names have not been changed.

natural_languages is now called language.
This attribute does not seem to be in use.

Add metadata serach fields for programming language
and keywords of a repository.

Keyword search only works for some words
at the moment, might be an issue with elasticsearch.

Signed-off-by: Lukas Kaltenbrunner <lukas.kaltenbrunner@student.uibk.ac.at>