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.

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>