Users can now edit the bookmark name and reminder time from their list of bookmarks.
We use "Custom" for the date and time in the modal because if the user set a reminder for "tomorrow" then edit the reminder "tomorrow", the definition of what "tomorrow" is has changed.
### UI Changes
If `SiteSetting.enable_bookmarks_with_reminders` is enabled:
* Clicking "Bookmark" on a topic will create a new Bookmark record instead of a post + user action
* Clicking "Clear Bookmarks" on a topic will delete all the new Bookmark records on a topic
* The topic bookmark buttons control the post bookmark flags correctly and vice-versa
Disabled selecting the "reminder type" for bookmarks in the UI because the backend functionality is not done yet (of sending users notifications etc.)
### Other Changes
* Added delete bookmark route (but no UI yet)
* Added a rake task to sync the old PostAction bookmarks to the new Bookmark table, which can be run as many times as we want for a site (it will not create duplicates).
