let's give this a shot

closes #71

* new route `/edit?url=` intended to be used as an edit button target, which detects the type of post and redirects to the appropriate editing interface
* implemented the edit interface for favorites

* now you can enter alt text for photos. see https://www.w3.org/TR/micropub/#uploading-a-photo-with-alt-text for details
* if the note has alt text, then the request is sent as JSON instead

closes #68

closes #69

update to "create" scope by default, but allow the user to choose "post" as a fallback. also updates indieauth/client to 0.2 for json support.

closes #67

* adds setting to choose the property to use for the syndication targets
* all existing users default to the old value `syndicate-to`
* new users default to `mp-syndicate-to`

closes #66

allows backdating and future-dating posts

closes #61

closes #62

Document supported URL parameters