Helper for dynamically generating the class=".." attribute.

Given a list like

[("a", True), ("b", False), ("c", True)]

builds a class name string for all parts where the second value is True.

E.g.

>>> classes [("a", True), ("b", False), ("c", True)]
"a c"

When setting b to True:

>>> classes [("a", True), ("b", True), ("c", True)]
"a b c"

Example:

>>> <div class={classes [("is-active", False)]}>
<div class="">

>>> <div class={classes [("is-active", True)]}>
<div class="is-active">

>>> forEach projects \project -> [hsx|
>>> <div class={classes [("project", True), ("active", project.active)]}>
>>> {project}
>>> </div>
>>> |]
If project is active:                        <div class="project active">{project}</div>
Otherwise:                                   <div class="project">{project}</div>