Use *base-directory* to find location of assets

Static assets are located in the `static/' sub-directory of the project.
Locating them with just `static/' works fine if execution is started in
the project's directory, but when starting somewhere else this causes
trouble.
Separate route from function definitions
Fix priorities for tasks

When setting and changing priorities no attention was paid to the
`story_id', which means that no two tasks could have the same priority,
even when belonging to other stories. This creates a weird situation
where the tasks of a certain story would become unmanageable because
there are gaps between the priorities. This could make it seem like
priority changing was impossible.