layouting

The new search-dropdown component HTML still
includes references to the search component.
Put these in a comment to allow compilation.
Since the component is not in use,
this problem only needs to be addressed if this
dropdown is used in the future.

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.

When clicking a local filter item
only its value is interesting.

Previously only the space taken up
by a filter item's name (e.g the name of the
repo "foo") was clickable.

Now the entire area highlighted on mouseover
can be clicked.

Instead of filtering by a single value,
use sets for each category.
Display results iff they match any set element
in all non-empty sets.