Uses the cl-prevalence system to store tasks on disk. The storage
location is either relative to the working directory in which the
system is started or determined (with priority) by the environment
variable `GEMMA_DATA_DIR`.
Adds a clickable area to the cards that will inform the backend of a
task being completed.
This of course still looks completely terrible because I don't really
know how frontend works.
The idea is that every task should be shown as an MDL "card" and have
some sort of associated action (probably more than just a click, but
that I'll look at ...).
Tasks are coloured based on their current "urgency".