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

- #1496: fallback plain text
- #1002: add date header
- #913: fix encoding of header

Signed-off-by: Unknwon <u@gogs.io>