Magic Folder: add join commandline tool

[dawuud]

For tahoe magic-folder join INVITATION LOCAL_DIR :

1. Parse INVITATION as COLLECTIVE_READCAP|CLIENT_WRITECAP.
2. Write CLIENT_WRITECAP to the file magic_folder_dircap under the client's private directory.
3. Write COLLECTIVE_READCAP to the file collective_dircap under the client's private directory.
4. Edit the client's tahoe.cfg to set [magic_folder] enabled = True and [magic_folder] local.directory = LOCAL_DIR.

[dawuud]

implemented the join command here in this dev branch ​https://github.com/david415/tahoe-lafs/tree/2454.magic-folder-create-tool.2

...however it's not quite done. It still needs to write config options to the tahoe.cfg

[dawuud]

completed. appends magic-folder options to tahoe config. needs unit tests.

[dawuud]

join unit test now tests that the collective read cap directory contains the readonly client dmd cap.

[daira]

Incidentally, we decided --after the design doc was written-- to use a different separator character than "|" between the COLLECTIVE_READCAP and CLIENT_WRITECAP, because "|" is a shell metacharacter (for a pipe, which could cause particularly confusing errors if unquoted). This is easy to fix.