feat: per-user auto directory creation

[ttys3]

Per-user auto directory creation

Description
as #657 mentioned
auto create user directory while creating new user.

Limitation

1. this logic DOES NOT work when edit an exists user.
2. this logic DOES NOT work when public user register on frontend page.
3. this logic ONLY WORK when admin is creating a new user on the user management page.

there are 2 conditions we need to consider
when creating a new user (with auto dir creation enabled):

1. user scope == global default user scope
2. user scope != global default user scope

condition 1.1

for example:
add new user foo and leave the scope dir to the default user scope . ,
and auto directory creation option was enabled,

suggest if we have the following startup command:

/path-to/filebrowser_amd64 -r /media

then the user's default scope dir will be:

./users/foo

and the real path on the system will be:

/media/users/foo

condition 1.2

another condition is the default user scope != server root path, but user scope = default user scope
for example:
we set the default user scope to ./bar under Global settings
when we add a user foo and auto directory creation option was enabled,
the user's default scope dir will be:

./bar/users/foo

and the real path on the system will be:

/media/bar/users/foo

condition 2

for example:
add new user foo and set the scope dir to ./special , and leave global settings default user scope still .
we do create the dir this time , but do not follow the default path creation logic.
the user's default scope dir will be:

./special

and the real path on the system will be:

/media/special

frontend assets PR is at filebrowser/frontend#102

[hacdias]

Please also add support for this option on config related commands and users new.

[ttys3]

config related commands and users new support has been added.

[hacdias]

LGTM! Thanks!