add task sign-up

[waymobetta]

add task sign-up for user account

need:

• task sign-up button
• API handler to accept task sign-up signal
• task signed-up/in-process status
• new table in db to manage state of the user's task

[waymobetta]

added sign-up button that redirects to specific task page

notes:

• doesn't need task state progress for now as tasks are very small and value remains unproven
• no api handler needed