repaired UI display of types and languages as it was again hidden behind the exercise cards on small screens

In the escape sequence "·" there was unicode
code point 0332 (UTF-8 cc b2), which is a
"combining low line". I think this does not belong
in there.

The exercise metadata component is only rendered
when it gets a value which is not null and has
a length of at least one (either string or array).
If it gets a value which is translated before it
is being passed, the result will be something of
the form "translation-not-found[...]", which has
a length of at least one, so it is rendered.
To prevent this, check if a value is set before
translating it.

They were replaced with exercise-metadata-item.

The goal of this component is abstracting away
the difference between metadata fields which are
strings and the ones which are arrays of strings.

This removes the <br> tags which were
previously inserted after every five elements.
Line breaks should be handled dynamically
by the browser.

This makes displaying metadata of an exercise
less verbose. Styling is a bit lacking as of now.

Restructure metadata translations of lists
to make declarations more concise.

Exercises should not depend on search,
so they get their own module.

This is done as to reduce the file size
and complexity of the exercise-details component.

Prior to this change, the image would
become full-width on narrow screens,
which looks awful.

Let the mailto link actually include
the email address instead of consisting
only of "mailto:"

This makes it look like the other buttons
in the modal, i.e. with an outline and white background.

The presence of "disabled" in a HTML tag
is sufficient to disable it.
Assigning a value has no effect.