Avoid duplication when adding online calendars
To visually make sense from a UI standpoint, chronologically, this soft-depends on the cleanup changes provided in !416. When you stack the two branches on top of each other, the puzzle pieces all "click" together.
This solves a long-standing annoyance I had with the current buggy workflow, and makes the UX at least predictable and "understandable". It doesn't try to fix everything under the sun, and the code diff is intentionally kept minimal to supersede @mcatanzaro's !129 (closed).
Basically what this does is: when subscribing to one or multiple online Webcal or CalDAV calendars, do not unexpectedly create an extra local calendar.
While this doesn't allow customizing names & colors while subscribing, users can still easily set those just after adding the subscriptions, as they are then returned to the "Calendars" page of "Manage Calendars".
Punting this to a separate step might not seem like the "perfect" flow, but it works well enough and fixes the buggy UX that we currently had.