* Add support for federated avatars

Fixes #3105

Removes avatar fetching duplication code
Adds an "Enable Federated Avatar" checkbox in user settings
(defaults to unchecked)

Moves avatar settings all in the same form, making
local and remote avatars mutually exclusive

Renames UploadAvatarForm to AvatarForm
as it's not anymore only for uploading

* Run gofmt on all modified files

* Move Avatar form in its own page

* Add go-libravatar dependency to vendor/ dir

Hopefully helps with accepting the contribution.
See also #3214

* Revert "Add go-libravatar dependency to vendor/ dir"

This reverts commit a8cb93ae64.

* Make federated avatar setting a global configuration

Removes the per-user setting

* Move avatar handling back to base tool, disable federated avatar in offline mode

* Format, handle error

* Properly set fallback host

* Use unsupported github.com mirror for importing go-libravatar

* Remove comment showing life exists outside of github.com

... pity, but contribution would not be accepted otherwise

* Use Combo for Get and Post methods over /avatar

* FEDERATED_AVATAR -> ENABLE_FEDERATED_AVATAR

* Fix persistance of federated avatar lookup checkbox at install time

* Federated Avatars -> Enable Federated Avatars

* Use len(string) == 0 instead of string == ""

* Move import line where it belong

See
https://github.com/Unknwon/go-code-convention/blob/master/en-US/import_packages.md

Pity the import url is still the unofficial one, but oh well...

* Save a line (and waste much more expensive time)

* Remove redundant parens

* Remove an empty line

* Remove empty lines

* Reorder lines to make diff smaller

* Remove another newline

Unknwon review got me start a fight against newlines

* Move DISABLE_GRAVATAR and ENABLE_FEDERATED_AVATAR after OFFLINE_MODE

On re-reading the diff I figured what Unknwon meant here:
https://github.com/gogits/gogs/pull/3320/files#r73741106

* Remove newlines that weren't there before my intervention

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