add vue recurrence picker

[jeriox]

closes #195 closes #958

• Implemented with Vue, similar to the signup block editor.
• Regarding the upcoming improvements in #958, the vue code is prepared to also pick times
• Getting the dates for the preview is no longer done via django, but directly in JS
• We decided against implementing a vue build step (see #1351)

ToDo

• [x] move widget to separate template
• [x] implement shift copy