Plan Notes: Allow users to add and edit their own notes

[matthiku]

• [x] create new table 'notes' linked to plans and users
• [x] Update 'addnote' modal popup to show all existing notes and allow editing and adding
• [x] JavaScript: update addNoteToPlan method incl. update to the DOM to properly reflect editing and adding of notes
• [x] Laravel: update PlanController
• [x] Add field to show that plan leader has read the notes (once he opens the modal popup)
• [x] Add notification on the plan page to show to the leader if there are unread notes for this plan
• [x] Leader's own notes are immediately marked as read & some layout improvements