Magic Folder: add invite commandline tool

[dawuud]

For "tahoe magic-folder invite COLLECTIVE_WRITECAP NICKNAME" :

(COLLECTIVE_WRITECAP can, as a special case, be an alias such as MAGIC:.)

1. Create an empty client DMD. Let its write URI be CLIENT_WRITECAP.
2. Diminish CLIENT_WRITECAP to CLIENT_READCAP, and diminish COLLECTIVE_WRITECAP to COLLECTIVE_READCAP.
3. Run "tahoe ln CLIENT_READCAP COLLECTIVE_WRITECAP/NICKNAME".
4. Print "COLLECTIVE_READCAP|CLIENT_WRITECAP" as the invitation, accompanied by instructions on how to accept the invitation and the need to send it over a secure channel.