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.

Clicking a repository has the following effect:
If the repository that currently is the filter
is clicked, the repository filter will be reset
to allow all repos.
If a different repo than the current filter is
clicked, the clicked repo will become the new filter.
This means that currently either all or exactly one
repo can be allowed in the filter.

All of the above also applies to file format.

The filter for university is not working
since IGitFiles does not have a corresponding field.

The user interface needs to be improved, since it
is not possible to see the current selections.

It might also be beneficial to allow multiple values
for one filter. (e.g. search for files that are either
.md or .java files).

The search component receives notifications
when the selected repository changes.
Upon such changes, only display the pages to which
the new filter applies.

The message service is used to transmit
information about a clicked repository
in the search information of the search page
to the search component.