Unix() already uses UTC as timezone

Sets all avatars to use PNG image format.
Keeps avatars consistent at 290x290px resolution.

Signed-off-by: Dennis Chen <barracks510@gmail.com>

Fixes relative urls coming from api/v1

See https://github.com/drone/drone/issues/1701

Also changes the avatar from a jpeg to a png, to allow for
transparent background. The indexed png is also smaller in size.

Note that at the moment the default avatar is only used when
the user requested a custom avatar and the custom avatar file
is not found (should never happen).

In the future the default avatar could be used as a default
return when by-mail avatar lookups fail too (both gravatar
and libravatar support passing a default)

Make user search function look in username (lower_name), full name
(full_name) and primary email (email). This will benefit searching after
user in "explore", admin panel and when adding new collaborators.

Both are possible on explore and admin panel

- Allow to delete current avatar

path.Dir can't handle Windows case, must use filepath.Dir

- Change tooltip size from mini to tiny in profile page

We set the default value for the non-NULL field NumFollowing of the User
model to 0, which stops an error when the ORM tries to sync.

- User profile un/follow
- List user's followers/following