RFC: Match by displayname, v2

Problem

Discovery has been a huge UX pain since forever.

• We require users to know identifiers that are neither well-defined nor exposed in the UI of any other tool for CardDAV/CalDAV.
• Local folders have two "names": The displayname file and the folder name. It doesn't make sense to have them as separate concepts.

Solution

In that order, ideally all at once:

• Match by displayname, see #420. Only use displayname in UI (collections arguments and "Syncing..."-output).
• Make sure to unicode-normalize displaynames to avoid "visual duplication".
• Allow arbitrary characters in collection names (internal property)
• DAV: Fix #405 by setting collection to an arbitrary unique identifier. Might as well be the entire URL.
  • Similarly for Google storage.
• filesystem: displayname == foldername for filesystem storage

Later

• Add option to DAV storage to ignore displaynames (for broken servers)

This is massive breakage. I'm unsure how a migration strategy for users could look like. Just tell the user to set up vdirsyncer from scratch?

Start by reviewing related issues #420 and #405, then trace how collection names and displaynames are represented across DAV, Google, and filesystem storage. Define the migration strategy and acceptance criteria for normalization, arbitrary identifiers, and UI output; the issue is done only when the breaking changes and upgrade path are resolved.