Overwriting an existing attachment file maliciously is possible since
Yona uses SHA-1 algorithm, which is known to be shattered[1], to digest
the contents to generate the names of the attachment files.

This fix ensures backward compatibility. Only new attachments have
filenames generated by SHA-256 algorithm. Fortunately, the length of
'name' column of 'attachment' table is 255 which is enough to store
SHA-256 which requires 64.

[1]: https://shattered.it/

See: Yona Github issue #178

See: Yona Github issue #180

- personal project = user's own private project

See: Yona Github issue #164

It can be set with "public" or "private".
Default value is "public"

eg.
project.default.scope.when.create = "public"

See: Yona Github issue #166

Notice
----
User profile page will be more simplified
in the future.

It's easy to slow down, but it's not used very well.

See: Yona Github issue #170

See: Yona Github issue #66

See: Yona Github issue #148