Dropbox enhancements

[tomt1664]

1. Get content hash via Dropbox API instead of download: https://www.dropbox.com/developers/reference/content-hash
   (this may require changing the way pymainstay hashes files).

2. Feature to create 'Mainstay' directory in the users dropbox account from the app. Also, when this directory is created, the 'base' TxID (i.e. result of msc info) should be saved to this directory in a plain text file.

3. Option to attest all the files in the 'Mainstay' directory (using the concatenation method defined by the -d argument in msc). i.e. get the content hash for each file in the 'Mainstay' directory, concatenate them (in alphanumeric filename order) and then attest the sha256 hash of this.

4. Option/button to download the full proof (i.e. msc fetch) from mainstay.xyz and save it to the 'Mainstay' directory in the dropbox account.