The project view crashed when accessing a task from /projects because the currentProject in store was not set, hence the views weren't set either. This change adds a fallback to it.
Related to #2246
Related to https://community.vikunja.io/t/vikunja-freezes/2246/5
Before this change, the filter button on the top right was positioned using absolute positioning and plenty of tricks, which were brittle and not really maintainable. Now, the buttons are positioned using flexbox, which should make this a lot more maintainable.